GB2079826A

GB2079826A - Underwater support structures for platform-carrying tower or towers

Info

Publication number: GB2079826A
Application number: GB8120448A
Authority: GB
Original assignee: Coyne et Bellier Bureau dIngenieurs Conseils SA
Current assignee: Tractebel Engineering SA
Priority date: 1980-07-09
Filing date: 1981-07-02
Publication date: 1982-01-27
Also published as: ES503712A0; ES8302153A1; FR2486562B1; BR8104399A; FR2486562A1; OA06851A; NO812313L; EP0046418A1

Description

SPECIFICATION Underwater support structures for platformcarrying tower or towers The present invention concerns mobile platform structures, particularly of the "self-lifting" type, designed to rest on a seabed, as well as a support structure particularly suitable for use with such a platform structure; the invention also concerns methods of anchoring and releasing such a platform structure. Platform structures which rest on a seabed by way of at least one pile or tower mounted on a support structure, have been in use for many years, particularly for the purposes of prospecting, drilling and production in marine oil-fields commonly known as "off-shore" operations. Furthermore, certain of these platform structures have been designed to be moved from one to another of the geographical areas in which work is to be carried out. Thus, since about 1953, use has been made of what are called self-lifting or self-jacking platforms comprising a barge which, during operations, rests on the selected seabed by way of piles which, at their lower ends, are mounted on a support structure and are displaceable in the vertical direction on the barge, so that the latter can transport them, when in a raised position in relation to the barge, to the selected site where the piles are lowered to provide support on the seabed. Different types of support structure are at present in use. Certain platforms rest on piles terminating in substantially cylindrical bases or caissons, the lower face of which is conical and which are usually referred to as "spud cans". Another type of support structure consists of a single sole-plate or closed caisson having dimensions comparable with those of the platform, which is then usually referred to as being "mat-supported". Depending upon the load-bearing capacity of the earth, that is to say of the seabed, these support structures either rest on the seabed or are buried in it. In the former case, the support structures at present in use cause no problems. In the latter case, the use of the known systems poses many problems that have not been completely solved. In particular, it is necessary to displace considerable quantities of material which have been removed from the seabed, and the burying of the support structure constitutes an operation which may have to be carried out, wholly or partly, in a violent manner which may hazard the stability of the structure as a whole. Furthermore, it is necessary to take into account, on the one hand, the safety regulations and specifications that are imposed, and, on the other, the risk of applying to the structure dynamic loads which are very much greater than the simple static loads (weight of the structure) and which result from natural phenomena, such as sea-swell, applying lateral stress to the piles and, consequently, to their support structure, so that it is found necessary to apply to the structure vertical forces which are at least equal to the maximum foreseeable forces. This is carried out by an operation of overloading the piles which is commonly known as "pre-loading" and in particular involves ballasting, which operation involves risks, causes loss of time and renders the installation of the platform structures more expensive. Finally, and more particularly in the case of self-lifting platform structures which, as previously stated, require to be moved from one work site to another, one of the most critical moments during the use of platforms resting on the seabed occurs when the support structure has to be disengaged from the seabed. In fact, after the support structure has sunk into the seabed, the latter becomes hard and compacted above the bases or caissons forming the structure, and it is then very difficult to extract the bases or caissons.In the case of a platform structure resting on the seabed by way of several piles, such disengagement of the support structure in the zone of certain of the piles may be attempted by applying force to other piles and this results in the application of very great forces to these other piles and there arises the risk of such force being concentrated on a small number of piles or even only one of them, since the centre of gravity is displaced. This results in the danger of break-up of excessively stressed piles and may lead to the platform structure turning over.Another solution, which may be used particularly in the case of a structure resting on the seabed by way of a single pile, consists in disengaging the support structure by clearing away the material (earth or sand), which covers it and holds it firmly in the seabed, this clearing operation being achieved by applying high-pressure water jets along the walls of the bases or caissons. The use of this procedure, known as "jetting", is time-consuming and costly, and is often found to be inadequate due to the fact that the support structure often becomes buried to a depth of several metres when the seabed is soft. This operation is not free from danger, and accidents have occurred with self-lifting platform structures during this stage of operations. Whatever the procedure used, the disengagement of such platform structures from the seabed on which they rest can be achieved only if wrenching forces, that may be very considerable, are applied, and this results in additional working costs and involves the superstructure and the personnel working thereon in considerable risks. French Patent 2335133 discloses a method and a marine support structure for use in a construction comprising a caisson having a floor or a closed base and provided with anchoring blades for securing the caisson when placed on the seabed. The caisson comprising a floor, which may carry a partition, is provided with anchoring blades which extend around the periphery of the floor and form a continuous wall which extends to or is located close to the lateral walls of the caisson, the partitioning of the floor by means of interior blades to form a compartment being also possible. After the caisson has been floated to the site and then completely submerged and lowered on to the seabed by ballasting, the anchoring blades pass through the sandy permeable surface layer of the seabed, then penetrate into the subjacent impermeable layer of clay, and co-operate with the floor or closed base of the caisson to form an enclosed space which is defined by the floor, the blades and the impermeable layer of clay and in which a vacuum is crested to cause the caisson to be buried more deeply and to anchor it by suction effect brought about by withdrawing and eliminating the water in the permeable sandy surface layer.As soon as the blades penetrate the seabed, the water enclosed below the floor can escape through vertical shaftes extending through the caisson, after which these shafts are closed by means of flaps, and removal of the water in the sandy layer is achieved by means of submerged pumps located at the lower part of hollow columns which likewise extend through the caisson and which connect the latter to a structure, such as a platform, located above the surface of the water. Two major disadvantages are associated with support structures equipped with such caissons which are of complex and therefore expensive construction since they are equipped not only with blades which form extensions of the vertical walls of a closed caisson, but also with vertical shafts, extending through the caisson and closeable by means of a valve, as well as with hollow vertical columns which likewise pass through the caisson. In the first place, these support structures permit anchoring only on seabeds having stratification layers that are thin as compared with the dimension of the floor and which comprise at least one permeable sandy layer resting on an impermeable layer of clay.This constitutes a considerable limitation in the use of these structures despite the fact that, in the above-mentioned French Patent, it is proposed to overcome this disavantage by depositing a layer of sand having the required surface on to the seabed before sinking the caisson. Furthermore, the vertical dimension of the blades, which is considerably less than the thickness of the caisson, must be greater than the thickness of the surface layer of sand. The dimensions of the support structure are therefore determined by the thickness of the layer of sand at the site. For these two reasons, such structures cannot be used for equipping self-lifting platform structures that are to be moved from one site to another, but only platform structures at a fixed location. The only possible remedy would be that of artificially reconstituting the seabed to suit it to each of the successive sites intended to receive a self-lifting platform structure. This would not of course be convenient and would call for lengthy and expensive large-scale engineering work. It should be pointed out that these banking operations are not always possible because of the depth of water at the selected sites. Futhermore, when use is made of the procedure in accordance with this French Patent, the sinking of such caissons into the seabed is limited to a depth that is very little greater than the vertical dimension of the anchoring blades. Sinking of the caissons to such a depth may however be insufficient, particularly when the seabed comprises a further layer of sand below the compressible layer of clay and if the latter is fairly thick. According to the above-mentioned French patent, the use of the above-described procedure must then involve other known means which for example, permit part of the compressible clay to be dredged by pumping and back-circulation into a series of pits distributed below the floor of the caisson and equipped with means for breaking up the clay.It is in fact obvious that after the anchoring blades have passed through the layer of sand, the support structure resists all further sinking by way of a surface which is that of the caisson assembly or the floor assembly and this would call for large-scale displacement of the material of the seabed as is the case when the support structure is of the mat-supported type which is described above and the disadvantages of which would occur in this instance. For the construction of dykes, breakwaters, moles, jetties, piers, wharves, seashore buttresses, booms, etc., United States Patent No. 3640075 has proposed the use of concrete caissons prefabricated for bringing into position and in the form of hollow caissons which are open at the bottom and may be partitioned and which have closed lateral walls having a vertical lower portion which, by way of an inclined planar or stepped upper portion, is joined to an upper wall which comprises orifices adapted to be connected to a pneumatic compressor or to a pump for handling solid materials. By controlling the pressure of a gas, such as air, in the caissons, these can be floated to the site, whereafter complete or partial discharge of the air, which is progressively replaced by a volume of water, enables the caissons to be lowered on to the seabed.The vertical portions of the side walls of the caissons can be embedded in the seabed to a controlled depth which is at most equal to the height of the vertical portions of the lateral walls; the attitude of the caissons may be controlled by using different pressures in the various compartments. Then, solid materials are pumped into the interior of the caissons, from which water is expelled, and the various openings in the upper wall, still above level, of the caissons are closed when the required anchoring position is reached. Such caissons may be released from the seabed and arranged in another position thereon by the reintroduction of air under pressure, this presupposing that the caissons are first at least partially emptied of their solid materials.So far, the use of such caissons has been envisaged only for providing structures projecting above the surface of the water and formed by the caissons themselves. Furthermore, for the purpose of sinking caissons, cylinders, tubes or other hollow structures having an open base into ground which may be on a seabed or on dry land, a procedure has been put forward in British Patent No. 288077, that consists in creating a partial vacuum in the hollow structure and then causing air or water to pass from the exterior to the interior of the hollow structure and below its lateral walls so as to loosen the ground supporting the walls so that the structure is able to bury itself in the ground under the effect of its own weight, which is supplemented by the effects of prdiessure applied to the hollow structure and by any additional loads that may occur.After the pressures within and outside the hollow structure have been balanced, a cover on its upper wall is opened and materials of the bed that at least partially fill the structure are removed therefrom to recreate an air chamber. The cover is then closed, and the vacuum is recreated in the hollow structure to permit further sinking. The present invention is aimed at overcoming the above-mentioned disadvantages of the known methods of and means for providing foundations for platform structures by using and adapting hollow caissons, open at the bottom, of the type considered in United States Patent No. 3640075 and British Patent No. 288077, for forming totally submerged supports for movable platforms, particularly of the self-lifting type, the form of such support or foundation structures being simpler and their cost lower than in the case of those hitherto used. In accordance with the invention there is provided a platform structure comprising a platform intended to rest on a seabed by way of one or more piles or towers mounted on a support structure, which support structure includes at least one caisson open at the bottom and having substantially vertical side walls, the upper portions of the caisson having at least one orifice having connected to it a duct, and a valve in the duct which is operable to control the flow of a liquid medium out of and into the interior of the caisson. The liquid medium is expelled from the caisson when the latter becomes buried by its side walls in the seabed for effecting anchorage of the platform structure, and enters the caisson when the latter becomes disengaged from the seabed to effect release of the platform structure. Thus, the foot of each pile or the common foot takes the form of a core sampler. An important advantage immediately results from this. In fact, the burying force depends only on the "pointed" surface of the vertical walls of the caisson and upon the friction along its vertical walls, since the liquid medium such as sea water from the adjacent area, which has been trapped during the descent of the caisson, can escape through the orifice or orifices provided in its upper wall. A single valve in a single duct may be provided into which run various branches connected to various orifices, or one duct and one valve may be provided for each orifice. Thus, by controlling the opening of the valve or valves, it is possible to vary the cross-section for the passage of the water and thus to regulate the discharge of water from the caisson, and in practice closure of the valve or valves arrests immediately the penetration of the caisson into the seabed. It is possible to increase the penetration of the caisson or caissons into the seabed, that is to say to go beyond the penetration normally obtained under the simple effect of the weight of the structure, with the aid of pumping means which are connected to the orifices by way of the ducts and which enable a pressure to be set up in the caisson that is lower than the hydrostatic pressure at the seabed. The pumping means is preferably such as to enable pressure to be set up in the caisson that is in excess of the hydrostatic pressure on the seabed thus assisting disengagement of the caissons from the seabed. By way of example, the load applied to the piles may be 100 tonnes, but the total area of the foot of the caissons may itself be in the order of approximately 100 m , so that quite low differences of pressure between the interior and exterior of the caissons enable considerable lifting and settling forces to be achieved. In one form of construction in accordance with the invention, a platform structure is so designed that its assembly of piles is mounted on a support structure comprising a single caisson which is open at the bottom and divided into compartments by internal vertical partitions, each compartment of the caisson having an orifice to which is connected a duct equipped with a valve for controlling the movement of liquid into and out of the compartment. In another construction a plurality of cais- sons are provided each supporting a single pile and each having at least one orifice to which is connected a duct provided with a valve for controlling the liquid movement into and out of the caisson. An expansible hydraulic chamber, bounded by an impermeable flexible diaphragm sealingly secured to the inner face of the upper wall of the caisson and/or on the vertical walls and/or, where provided, partitions of the caisson, is formed within the caisson or within each of its compartments and can be emptied or filled, at least partially, through the orifice or orifices so as respectively to promote the penetration of the caisson into the seabed or its disengagement therefrom. In a platform structure of the self-lifting type, whereof the piles are intended to support a barge on which they are mounted to be movable in the vertical direction, the valves are preferably mounted on the barge. Furthermore, these valves are preferably connected to a pump unit which is likewise mounted on the barge and which performs the function of setting up reduced pressure or excess pressure in the caisson or caissons with respect to the pressure acting externally on the caisson.The invention also provides a support structure for a platform structure projecting above a water surface and intended to rest on a seabed which support structure comprises at least one caisson open at the bottom and having substantially vertical side walls, each caisson having defined within it at least one enclosed volume which is expansible and contractable, and means for pumping liquid selectively into and out of said volume. The method of anchoring a platform in accordance with the invention comprises the steps of immersing the support structure until the lower ends of the vertical walls of the caisson or caissons come into contact with the seabed, regulating the rate of flow through said orifice or orifices and/or the pressure in the caisson or caissons to control the rate and/or the depth of penetration and the attitude of the caisson or caissons in the seabed, and closing the orifices when the required penetration is achieved. To assist penetration, the method preferably also comprises setting up a reduced pressure within the caisson or caissons. When it is to be used on a very permeable seabed, the method in accordance with the invention preferably comprises prior to commencement of penetration of the caisson or caissons into the seabed, the step of filling the interior of the caisson or caissons with a muddy medium and then regulating the discharge of this muddy medium from the caisson or caissons by means of the orifices so as to control penetration. In the case where each caisson of the support structure of a platform is provided with an expansible hydraulic chamber, the method further comprises the step of filling each of said expansible hydraulic chambers with a liquid medium for submerging the support structure and then regulating the discharge of the liquid medium from each cham- ' ber so as to control the penetration.For the purpose of releasing the support structure of a platform structure of the invention when anchored on a seabed, the method in accordance with the invention comprises the steps of introducing a liquid medium through said orifice or orifices so as to set up in the caisson or each caisson a hydraulic excess pressure adapted to raise the caisson or caissons from the seabed, and regulating this excess pressure so as to control the rate and/or the extent of the disengagement of the caisson or caissons from the seabed. When the seabed is very permeable, it is preferred to introduce a muddy medium into each caisson. If each caisson is equipped with an extensible hydraulic chamber, a liquid medium is introduced into each chamber at a rate which is regulated to ensure controlled release of the caisson from the seabed.This preferred method is achieved by means of a support structure comprising an improved construction of the caisson having an open base and vertical lateral walls and possibly divided into compartments by vertical partitions and the upper wall of which has at least one orifice for the passage of a liquid medium, which is expelled from the caisson or caissons during penetration into the seabed and which enters the caisson or caissons during release from the seabed, at least one body having a volume which can be varied being retained in each caisson and if necessary in each of its compartments (when provided), and this body being compressed during penetration and expanded during release. Preferably, the said body is formed by an expansible hydraulic chamber bounded by an impermeable diaphragm secured on the inner face of the upper wall and/or on the vertical walls and partitions of the caisson or caissons and communicating with the orifices. Further features and advantages of the invention will emerge from the following description of embodiments of the invention illustrated in the drawings, wherein: Figure 1 illustrates a self-lifting platform mounted on a single caisson, Figure 2 illustrates a self-lifting platform on which each pile is supported by a separate open caisson, Figure 3 shows a platform which is mounted on a single caisson having orifices which are connected to a system of ducts and valves, Figure 4 illustrates a platform of the same type as shown in Fig.3, but with a separate caisson for each pile, Figure 5 is a plan view of a caisson designed to support all the piles of a platform, this caisson being of the partitioned type, Figure 6 is a diagrammatic view of a caisson equipped with an expansible chamber, and Figure 7 shows a diagrammatic view of a partitioned caisson, each compartment of which is provided with an expansible chamber. The drawings illustrate only two piles for each platform, but it will be understood that this does not constitute any limitation whatsoever. Fig. 1 shows the barge 1 of a self-lifting platform supported on lattice-work piles or towers 2 resting on a single hollow caisson 3, which is open at the base and which has in its upper wall a number of orifices 3', to which are connected ducts 2 provided with valves 4 advantageously controlled from the barge 1. The barge 1 itself carries drilling or production means 6. When the caisson 3 reaches the surface A of the seabed, the vertical lateral walls 5 of the caisson 3 penetrate into the seabed, and the initially trapped water escapes through the orifices 3' and ducts 7, as long as these remain open. When the valves 4 are closed, water that may still be trapped in the caisson 3 continues to be held within the caisson, and downward movement of the latter immediately ceases. It should be pointed out that the load-supporting capacity of the foundation at this moment is equal to that of a sole-plate having the same section and disposed at the lower level reached by the caisson 3, and acquires a value greater than the force required for embedding the caisson in the case where the valves 4 are open.The quantity of material displaced, corresponding to the volume of the vertical walls of the caisson, is therefore small, and penetration of the caisson does not cause any largescale displacement of the material below the foundation. This constitutes a considerable advantage as compared with fully-walled caissons or sole-plates, which can penetrate the bed only by breaking it up and displacing it, since this leads to a reduction in the mechanical strength of the seabed. When embedding of the caisson 3 is complete, its load-supporting capacity is very much greater than the static load, so that superimposed dynamic loads can be supported with adequate safety. Fig. 1 therefore illustrates the caisson either during its penetration into the seabed or upon completion of penetration to an intermediate position, the valves 4 being closed although the caisson is not completely embedded in the seabed. Referring now to Fig. 2, each of the piles or towers 2 of the platform structure is mounted on a separate caisson 3, the upper face of which contains two orifices 3', and in this instance no duct or valve is provided. In Fig. 3, the piles or towers 2 are again shown mounted on a single caisson 3, but valves 4 are provided on the barge 1 and are each connected, on the one hand, through ducts 7 to several orifices 3' formed in the upper wall of the caisson 3, and on the other hand, to a pumping unit 12 which can be used to enable reduced pressure to be set up within the caisson 3 to promote its penetration into the bed, and to enable excess pressure to be set up in the caisson 3 to permit it to be disengaged from the seabed. Referring now to Fig. 4, each pile 2 is shown supported by a separate caisson 3, the upper end of the space within which communicates, by way of the ducts 7 and 8 each connected to an orifice 3', with valves 4 carried by the barge 1. These valves 4 are, of course, preferably connected, as in the construction of Fig. 3, to a pumping unit (not illustrated). Fig. 5 illustrates, in plan view, a caisson in accordance with the invention and of circular and compartmented form. This caisson is divided into sectors by solid vertical partitions 9 and 10, said sectors in turn being divided into two parts by a solid vertically extending ring 1 1 . This division is made for the obvious reason of increasing the mechanical strength of the caisson. Each of the compartments thus formed is provided, as explained above, with an orifice 3', to which is connected a duct provided with a valve, conveniently operated from the barge 1. This enables uniform penetration of the caisson into the seabed to be achieved by appropriate operation of the valves. The anchoring and the disengagement or release of the various platforms, the foundation structures of which have been described by reference to Figs. 1 to 5, are achieved as follows: for anchoring-immersing the foundation structures, then regulating the output by way of the orifices 3' and/or the pressure in the caisson or caissons 3, which may be a reduced pressure, so as to control the rate and/or the depth of penetration (and if necesssary to promote penetration) as well as for varying the attitude of the caisson or caissons 3, then closing the passage orifices 3' when the required depth of penetration is achieved; for effecting disengagement-introducing a liquid medium, such as water from the surrounding area, into the caisson or caissons 3, through the passage orifices 3', and setting up, in each caisson 3, excess hydraulic pressure so as to lift the caisson from the se bed, and then regulating this excess pressure to control the rate and/or extent of the disengagement of the caisson 3 from the seabed. However, if the seabed is very permeable, it is preferable to make use of a muddy medium instead of the water taken from the surrounding area, for the purpose, on the one hand, of filling each caisson 3 before beginning to embed it, then regulating the discharge of this muddy medium from the caisson 3 through the passage orifices 3' so as to control the depth of penetration of the caisson, and, on the other hand, for introducing the medium into each caisson to obtain the excess pressure that enables it to be disengaged, since the use of a suitable muddy medium enables the forces applied to the seabed to be suitably distributed, and renders it possible to avoid any premature penetration of the caisson into the seabed as a result of percolation of water in the bed. Alternatively, in an arrangement which is suitable for seabeds, whatever their permeability, a body having a variable volume is held in the caisson 3 or within each of its compartments, and this body is able to contract from a given initial condition when the caisson penetrates the seabed, adjustments of its contraction or reduction of volume permitting, if required, control of penetration as regards rate, depth and/or attitude. This body may be disposable and, if necessary, destroyed during penetration of the caisson.Release of the caisson may be achieved by introducing into it a liquid medium (water or mud) as explained above, but the said body may also be of a construction which enables it to expand or increase in volume from another given initial condition, so as to permit release of the caisson to be controlled by regulating the increase in volume of this body, and a second given initial condition may be that of the body on completion of the preceding embedding operation. Fig. 6 illustrates diagrammatically a first form of construction of such body of variable volume; this body is formed by an expansible hydraulic chamber bounded by a flexible, deformable, fluid-tight diaphragm 12, which is fully enclosed except for an opening through which the chamber communicates with the orifice 3' of a caisson 3.The diaphragm is secured, for example, bonding, vulcanization, glueing or any other suitable means, to the lower surface of the upper wall of the caisson 3. The fluid-tight chamber of variable volume, fitted in this way in the caisson, may be filled with a liquid, which may be water pumped from the surrounding area, or a liquid selected for the purpose of reducing or preventing corrosion of the pumping unit, for example. This liquid is used to cause the caisson 3 to be immersed when the platform is anchored; the discharge of this liquid through the orifice 3' can be regulated when the lateral walls 5 of the caisson 3 penetrate the seabed, so as to control this penetration.When the caisson is to be raised, this liquid can be introduced, on a regulated basis, into the chamber, which has been at least partially emptied during the embedding operation, so that a pressure in excess of the hydrostatic pressure is set up. The chamber increases in volume and thus pressurises the materials contained in the space 13 below the diaphragm 12 in the caisson 3, so that the caisson is lifted and disengaged from the seabed by reaction on these materials (which may be water or substances from the seabed of the seabed itself. In this arrangement it is of course also possible, by pumping, to set up reduced pressure in the space within the filled diaphragm 12 so as to promote penetration into the seabed. This arrangement offers the advantages that the liquid of the control circuit is isolated by the diaphragm 12 from the surrounding medium if the ducts 7 discharge into surface reservoirs provided for this purpose on the barge 1, for example, while on the other hand, it is possible, by discharging this liquid and filling the chamber with a pressurized gas, to displace the caisson 3 alone, or in association with a pile 2 or the barge 1, by flotation if necessary. Fig. 7 illustrates a caisson 3 divided into compartments by means of vertical partitions 11 and the lateral vertical walls of the caisson 3. By way of orifice 3, a duct 7 terminates in a closed chamber of variable volume, each compartment of which is delimited by a flexible, deformable, impermeable diaphragm 14, sealingly secured along the lower edges of the walls 5 and the partitions 11 which bound the corresponding compartment.The mode of operation and advantages of this caisson 3 are in all respects similar to those of the caisson illustrated in Fig. 6, except that stabilization of the caisson 3 can also be controlled by differential regulation of the expansion and/or expansion of the various chambers, disengagement of the caisson 3 from the seabed being achieved by reaction on the materials contained in the spaces 15 below the diaphragms 14, when the chambers of the caisson 3 are pressurized. It will be understood that, without departing from the ambit of the present invention, the forms of construction that have been described can be modified, in particular by the substitution of equivalent technical means.

Claims

1. A platform structure comprising a platform intended to rest on a seabed by way of one or more piles or towers mounted on a support structure, which support structure includes at least one caisson open at the bottom and having substantially vertical side walls, the upper portion of the caisson having at least one orifice having connnected to it a duct, and a valve in the duct which is operable to control the flow of a liquid medium out of and into the interior of the caisson.

2. A platform structure according to Claim 1 further comprising pumping means connected through said ducts to the interior of the caisson for enabling a controlled pressure to be set up in the caisson which pressure is selectively lesser or greater than the hydrostatic pressure on the seabed so as respectively to promote penetration of the caisson into the seabed and disengagement of the caisson from the seabed.

3. A platform structure according to Claim 1 or Claim 2, wherein a plurality of said caissons is provided each of which has mounted thereon a single pile or tower, the piles or towers jointly carrying the platform.

4. A platform structure according to any one of Claims 1 to 3, wherein an expansible hydraulic chamber, bounded by an impermeable flexible diaphragm sealed with respect to the inner face of the top wall of the caisson is provided within the caisson and can be selectively emptied and filled, at least partially, through said orifice or orifices so as respectively to promote penetration of the caisson into the seabed and its disengagement therefrom.

5. A platform structure as claimed in any one of Claims 1 to 3, wherein the interior of the caisson is divided into separate compartments by vertical partitions, each compartment having a valve-controlled duct opening through an orifice to the upper portion of the compartment.

6. A platform structure as claimed in Claim 5, wherein an expansible hydraulic chamber bounded by an impermeable flexible diaphragm sealed with respect to the inner face of the top wall of the caisson is provided within each of said compartments and can be selectively emptied and filled, at least partially, through the orifice opening to the compartment so as respectively to promote penetration of the caisson into the seabed and its disengagement therefrom.

7. A platform structure according to any one of Claims 1 to 6, and of the self-lifting type, the platform of which is constituted by a barge on which the pile or piles are mounted to be movable in the vertical direction, wherein the valve or valves are mounted on the barge.

8. A method of anchoring a platform structure as claimed in any one of Claims 1 to 7, which method comprises the steps of immersing the support structure until the lower ends of the vertical walls of the caisson or caissons come into contact with the seabed, regulating the rate of flow through said orifice or orifices and/or the pressure in the caisson or caissons to control the rate and/or the depth of penetration and the attitude of the caisson or caissons in the seabed, and closing the orifices when the required penetration is achieved.

9. A method according to Claim 8, comprising the step of setting up a reduced pressure within the caisson or caissons in order to promote penetration.

10. A method according to Claim 8 or Claim 9, comprising, prior to commencement of penetration of the caisson into the seabed, the step of filling the interior of the caisson with a muddy medium and then regulating the discharge of this muddy medium from the caisson or caissons by means of the orifices so as to control penetration.

11. A method according to Claim 8 or Claim 9 for use with a platform structure as claimed in Claim 6, comprising the step of filling each of said expansible hydraulic chambers with a liquid medium for submerging the support structure and then regulating the discharge of the liquid medium from each chamber so as to control the penetration.

12. A method of disengaging a platform structure as claimed in any one of Claims 1 to 6 from a seabed in which the platform has been previously anchored, comprising the steps of introducing a liquid medium through said orifice or orifices so as to set up in the caisson or each caisson a hydraulic excess pressure adapted to raise the caisson or caissons from the seabed, and regulating this excess pressure so as to control the rate and/or the extent of the disengagement of the caisson or caissons from the seabed.

13. A method according to Claim 11, comprising the step of introducing a muddy medium into each caisson.

14. A method according to Claim 11 for use with a platform structure as claimed in claim 6, comprising the step of introducing a liquid medium into each of said hydraulic chambers to expand the chambers and cause the caissen or caissons to disengage from the seabed.

15. A support structure for a platform structure projecting above a water surface and intended to rest on a seabed which support structure comprises at least one caisson open at the bottom and having substantially vertical side walls, each caisson having defined within it at least one enclosed volume which is expansible and contractable, and means for pumping liquid selectively into and out of said volume.

16. A structure according to Claim 14,wherein said enclosed volume is bounded by an impermeable flexible diaphragm which is sealingly secured to walls of the caisson and an orifice opening through a wall of the caisson to said volume, the pumping means being connected to said orifice.

17. A platform structure substantially as hereinbefore described with reference to and as illustrated in any one of Figs. 1 to 7 of the accompanying drawings.