referring to fig1 to 4 the structure comprises three columns 1 braced together by horizontal braces 2 and 3 with cross braces 4 , each of the columns having a foot 5 . the general concept of the structure is that although the columns may have to be very long , they can be constructed in a horizontal position and the structure can be floated in shallow water with only one side submerged so that the whole structure can be towed in the horizontal position floating on the water to the site , and then upended and sunk onto the sea bed with the upper end rising above the surface of the waves at the site . each column and bracing member ( except the top bracing which is out of the water when the tower is being upended ) is a circular cylinder of a diameter large enough to provide the required buoyancy and water plane area to float in shallow water and to maintain floating stability during all stages of the towing to the site and the upending and sinking steps . the three columns 1 are joined together by two horizontal bracing systems consisting of brace members 2 at the bottom of the structure and 3 at the top of the structure . each of these bracing systems comprises the horizontal members 2 and 3 , there being a member 2 and 3 extending between each column so that in plan in the erected structure the systems 2 , 3 are triangular with the columns at the angle . a secondary set of bracing members , 2a , forming a triangle of half size and connecting the middle points of the three bracing members 2 may be added in order to strengthen the bracing and side support to conductor guides and various pipe connections rising from the sea bed . each column is made tubular and may be of reinforced or prestressed concrete or steel . it has a central sleeve or duct through it shown at 1a in fig6 and 7 for the purpose to be described ; alternatively each column may have vertically spaced horizontal diaphragms 1b ( fig1 a ) with central holes 1c for the same purpose . the columns are interbraced between their ends by the diagonal bracing members 4 shown in the form of a cross but they can be arranged in any geometrical configuration to ensure that the whole stability of the structure is maintained with a minimum number of bracing members , having in mind that the main aim is to achieve a structure which offers minimum resistance to wave action and has no horizontal members in the critical wave zone . the bracing members 4 may be of steel or of other rigid material such as reinforced or prestressed concrete but as shown in fig3 they may be hollow and can be flooded with water during the sinking operation . functioning as diagonal bracing members they will during wave action on the erected structure be subject to tension and compression forces of equal magnitude , the bracing members 4 may preferably be made of steel , which also gives them additional natural buoyancy which is an advantage during the tow - out and upending operations . the structure thus described is made to float in the manner shown in fig3 so that the two columns indicated at the bottom of fig3 plus their corresponding bracing members 2 , 4 are sufficiently buoyant to hold the whole structure on the surface of the water . if desired a superstructure or deck structure may be built onto the top end of the columns at the inshore construction site , in order to avoid erecting the platform or superstructure on the tower once it has been established on the sea bed . this whole operation is reversible so that the structure can be raised and towed to the new site . for this purpose buoyancy tanks 7 can be temporarily attached to the floating structure and thereby give added buoyancy at the top end of the structure which will counteract the downward forces arising from the weight of the superstructure attached to the main structure . these buoyancy tanks can be provided with means for flooding and deballasting should this be necessary during the upending and sinking operation . with this appraoch it is possible to install any amount of deck load before tow - out without running into floating stability problems . the temporary buoyancy tank remain close to the surface . they are therefore not subject to great hydrostatic pressures and consequently of quite ordinary design . any amount of drilling and production equipment can be installed before towing the structure to site , as long as the items in question can accept a gentle rotation through 90 ° during the upending movement . the feet 5 each comprise a conical section 8 which is surrounded at its bottom end by a circular sleeve 9 connected at 10 to the outside of the cone 8 . the shape of the feet , i . e ., the bottom diameter , the height of the sleeve 9 and the overall height of the conical section 8 , is such as to make it suitable for the conditions of the sea bed at the site as determined by the sea bed soil survey . the sleeve , tube or hole 1a in the column 1 opens out into the top of the conical part 8 through which a pipe 11 can be passed which may carry either a suction pipe or form a suction pipe and have at its lower end one or more cutter heads 12 . if desired the suction pipe and the pipe carrying the cutter 12 could be separate pipes which run side by side in the sleeve 1a or they could be withdrawn and replaced one for the other . the sleeve or hole 1a should preferably have such a large diameter that a diver or a diving bell possibly could pass down into the foot if something completely unforeseen should happen , or enable operators to inspect , test or sample the sea bed in or beneath the foot before backfilling . it is important to be able to guide the cutter - suction head to any part within the foot and at that part exert a force of a substantial magnitude as required to carry out the excavation process effectively . fig8 to 10 show one way of achieving this objective . a horizontal guide rail 14 for two movable anchor blocks 15 is fixed to the conical wall 8 . the two anchor blocks 15 are placed at diametrically opposite points on the guide rail . each block is kept in position by a wire rope 16 running on the inside of the guide rail and through a number of guide tubes 17 taken up to a winch on the deck . by a simple winching operation it is then possible to locate these two anchor blocks anywhere along the perimeter of the conical wall . another set of wire ropes 18 connect the suction pipe 11 to the two anchor blocks and another simple winching operation from the deck enables the cutter - suction head to reach any point on the diameter between the two anchor blocks and at each point exerting a force governed by the weight of the cutter - suction assembly ( including the weight of the pipe ) and the winching force applied . the proposed structure could be constructed in a number of ways . fig1 shows a typical two - stage construction . the first stage is to construct one side of the structure ; i . e ., two columns with their interconnecting bracing members , in a purpose made dry dock . without the feet this part of the structure can float in less than 5m of water , so the dry dock may be so shallow that it can be established without difficulty in most ground conditions . having completed the first stage construction the dry dock is flooded and the structure is floated to a sheltered place where it can be completed while floating . a suitable construction method to adopt for this second stage is the free cantilever construction method generally used for major concrete bridges spanning over wide rivers or crossing inaccessible or soft ground . the 2 feet may be constructed in the water line as two separate units , floated into position and stressed to the columns . the third foot on the column may then be constructed in - situ using the cantilever method . the buoyancy tanks 7 , if used , are then attached to one or both of the floating columns and finally the superstructure indicated at 13 in fig1 is then built onto the structure . the structure thus built is capable of floating in shallow water . ignoring the feet it is quite feasible to construct a structure , say 1 , 000 feet long which will float in 10 - 12m water depth . taking the feet into account , it is likely that the required water depth would be 20 - 25m , but this is still quite shallow and can be found on many locations . the structure can then be towed to the site where the sinking operation is to be carried out . as seen in fig4 the sinking operation is carried out by flooding the columns 1 and the lower brace members 2 and as an optional feature the braces 4 or part of them . each of these parts of the structure has appropriate valve means controlled from the top end of the structure to control the inflow of water . as can be seen from fig4 the flooding of the structure members will start at the lower end of the final structure which is in the water and thus the structure begins to tilt in the water and at the same time to sink . the tilting can be controlled to any speed required and can be stopped at any time or reversed . as the flooding continues the structure will gradually move into an upright position and float vertically without any of the feet touching the sea bed or sink until two of the feet 5 engage the sea bed . by continuing flooding the structure will touch down on all three feet more or less simultaneously dpending on the sea bed topography or by continuing the flooding of the third column the foot of which is not yet in contact with the sea bed , and possibly by deballasting the two other columns or the buoyancy tank , the structure is brought into an upright position with all three feet engaging the sea bed . at the juncture the cutter head is brought into operation as seen in fig5 to cut away the material of the sea bed which is then sucked up through the pipe 11 and discharged into the surrounding water or it may be discharged into floating pontoons or like structures for use in backfilling of the feet at a later period of the operation . as the cutter head cuts away the sea bed within the conical foot 8 the foot sinks into the ground and this operation continues until all the feet are firmly based in the sea bed with the column vertical . finally , when the feet have reached the required foundation they are secured at that depth by backfilling the conical space underneath the feet with granular material such as sand or gravel and if need be the hollow space inside the feet and part of the column and / or the braces . the backfill of coarse granular material underneath the feet serves a double purpose of making it possible to drain the feet and controlling the pore water pressure underneath the structure as well as facilitating a recovery of the structure after it has served its useful life in this particular site location . the superstructure 13 and the deck installations are then completed insofar as is necessary and the platform is ready for the operation for which it is intended . referring to fig1 to 21 these show an alternative construction similar in principle to the construction described with reference to fig1 to 13a . in this alternative construction the structure is constructed as two separate units , the first forming a basic structure with the braced legs 1 and the second forming the superstructure 20 . the superstructure 20 is constructed to be buoyant so that it can be floated and towed to the site separately from the basic structure and preferably as shown in fig1 complete with drilling and production equipment and facilities ; it can be mounted on the erected basic structure at the site by a conventional self - contained jack - up procedure so as to be above the highest predictable waves at the site . the superstructure can form the upper horizontal bracing member 3 ( fig2 - 4 ). fig1 shows another form of foot 5 having a conical section 8 enclosed within a hollow sleeve 9 and roof 10 of greater size than the construction in fig7 and 8 . this construction gives the foot more natural buoyancy in the tow - out and reduces the final ground pressure underneath the foot when finally installed in the sea bed than the form shown in fig7 and 8 . fig1 and 16 show an alternative construction of means of excavating the sea bed within the foot . the leg or column 1 has within it a pipe 11 extending downwardly through which passes a suction pipe 22 branching out at its lower end within the foot 5 into the suction pipes 23 , 24 each carrying a cutter 25 at its end . the upper end of the pipe 22 is connected at 26 to a source of reduced pressure . a lifting tackle on the superstructure 20 includes a wire or cable 27 extending down through the pipe 11 and connected at its lower end to toggles 28 pivoted to the pipes 23 , 24 whereby the pipes 23 , 24 can be spaced apart to move the cutters 25 into engagement with the sea bed round the periphery of the foot 5 . the cutters may be rotatable or otherwise operable by controls passing down the suction pipes from the superstructure to excavate the sea bed within the foot , the resultant debris being sucked up through the pipe 22 . the cutting action can be enhanced by the downward thrust of the pipes 22 , 23 , 24 by gravity due to the weight of the cutter and pipe assembly or the assembly may be forced downwardly by suitable means controlled from the superstructure . the natural action of the pipes 23 , 24 is to spread apart under their own weight so that the excavation is carried out by a series of radial cuts of the cutters , the pipes 23 , 24 being rotated within the foot when the weight of the assembly is supported off the sea bed by means controlled from the superstructure . alternatively the pipe 22 may extend downwardly between the toggles 28 and the pipes 23 , 24 may then be rods carrying radially inwardly directed drag teeth so that the toggles are actuated to spread the rods and then draw them back with the teeth drawing debris to the center where the suction pipe removes it . in another way of constructing the structure the upper cross bracings 3 are prefabricated out of steel , and erected in a dry dock and connected to the ends of the columns 1 as they are being constructed , starting from their top ends . as the lengths of the columns 1 is being built up the next set of cross bracings , i . e ., the top bracings 4 , are connected into the columns and this process is repeated until the structure is complete but without the feet 5 or the superstructure 20 . at the same time the three feet 5 can be constructed for example vertically and upside down working upwardly . the structure and the feet are floated into shallow water and the feet are mounted on the lower ends of the columns 1 as by cementing or concreting by conventional methods . the feet and structure may be united with a final prestressing operation in a conventional manner . it will be understood that by constructing the structure in the manner described above in a horizontal position the design and construction of the feet can be left until a late stage in the overall program enabling up - to - date survey of the sea bed to be used in designing and constructing the feet 5 to suit the sea bed conditions . with the feet well buried in the sea bed and the possibility of draining the feet properly it is possible to take into account the anchoring effect against the cyclic wave loads operating against the structure and thus be able to make the structure considerably lighter than more conventional structures at present being built . it will be understood that weight can be added to the structure by pumping the amount of ballast required into the hollow feet , the columns and / or bracing members . compared with existing known structures erected or being built the structure of the present invention is considerably lighter indicating that the material from which it is made , be it steel , reinforced or prestressed concrete has been used to good advantage . furthermore , due to its lightness and considerable natural buoyancy , the differential water pressure between water within and outside the structure members on any of the members during the sinking and upending process can be kept within such limits that this loading case becomes far less severe than for the structures presently being built , where the compression due to external water pressure often governs the wall thicknesses . it also enables the structure to be built in shallower water than known structures and because of its smaller mass and because it is floating horizontally and therefore is less exposed to wind forces , it requires less tugboat power to be towed and kept under control . the method of digging the three feet into the sea bed to reach a reliable foundation level provides a good foundation system for the structure even where the sea bed conditions and topography are not well defined . it is a major advantage that any amount of deck load and equipment can be applied to the structure in sheltered water before the towing starts , permitting drilling or other operations for which the structure is intended to start immediately it has been safely founded in the sea bed in a matter of a few days after touching down . it is possibly an even greater advantage that the structure naturally divides into two structures with the top part being a completely independent jack - up platform , the installation of which only requires a very short spell of calm weather ( 3 to 4 hours ). this not only promises a quick start on the oil drilling and production , but it promises an equally quick dismantling and start somewhere else . moreover the structure is comparatively independent of weather conditions even during the upending and sinking operations since this operation is of short duration , 8 - 10 hours , and the structure becomes less and less vulnerable to winds and waves as the operation proceeds . this could lead to such confidence in the safety of the offshore operation that one might consider installing the structure during even the shortest weather window , that is the required starting conditions and a reasonable 12 - hour weather forecast . finally , since the whole operation can be halted at any stage and even reversed it is also possible at a later stage to recover the structure by deballasting and refloating it .