NO142535B - FRONT DRAWING FOR GREAT DEPTHS. - Google Patents

Description

The invention relates to seabed-supported constructions that enable the extraction and storage of oil in very deep water, i.e. in the area from approx. 90 - 365 m or more, in accordance with the introduction of the patent claims.

Until now, piers, designed for stability standing on the seabed in water deeper than approx. 30 m, has been proposed for the storage of liquid materials in a central tank or a group of tanks with vertical axes, supported by a plinth plate and extending beyond the range of the gravity waves of greatest amplitude propagating over the location. These piers have required the construction of a wall that surrounds the entire periphery and is able to convert the total energy of a wave into exclusively kinetic flow energy in seawater, guided as jets by channels in the form of through holes in the wall.

The divided vessel according to US-PS 3,858,402 with valve devices^ provides access for seawater to a lower-lying space, while air is discharged to the atmosphere to cause an inclined position in relation to the horizontal floating position. The side compartments are then vented to achieve lowering of the vessel. However, no precautions have been taken, not even in the case of vessels with long slender shafts to be placed on the seabed in deep water, to coordinate internal pressure with sea pressure at a medium depth.

US-PS 3,383,869 describes marine pier structures with sea-stable permanent columns standing on the seabed, characterized by a continuous circumferential, perforated outer wall and a continuous inner wall spaced from and attached to the outer wall and with a central space. Such columns are designed as floating forms which are sunk into place in an upright position and do not include compartmentalized spaces for this use.

US-PS 3 886 753 describes a partially submersible tower construction with a lower group of cells spaced apart for free water flow and a series of submerged tanks on the lower part which are loaded with sand after submersion of the tower on the seabed. A central room extends over the sea and contains oil. Initial submersion removes air from the tanks using seawater, as the air escapes to the atmosphere. However, precautions have not been taken to coordinate the pressure in the room with the sea pressure at a specific depth, particularly at a medium height in a vertically elongated room.

US-PS 3,921,408 does not disclose submerged tanks and indications are given to protect a tank wall against larger pressure differences.

GB-PS 1 354 258 describes a multi-tank construction which, when lowering begins, is gradually supplied with pressure from air, the lowering being stopped at intervals for pressure equalisation. However, no measures are mentioned for regulating the pressure in the upper part in relation to the depth, either during immersion or during use.

Neither GB-PS 1 048 775 nor GB-PS 1 04 7 776 disclose measures for equalizing pressure in ballastable chambers in the hull with sea pressure under any ballast situation, apart from creating an open upper sea surface. In the enclosed submerged horizontal ballast pipe, the space is filled with fluid of unspecified pressure.

The derrick construction according to US-PS 3,875,371 has no submerged tanks and includes no perforated vertical walls and thereby differs from the objective of the present invention.

An off-shore bottom-standing tower as shown in FR-PS 2 251 208 exhibits no adjustment of the gas pressure in a gas-filled submerged chamber which should be approximately at the mean hydrostatic sea pressure over the vertical extent of the chamber which is not filled with ballast water.

The fabrication in connection with the known technique requires an initial construction phase in dry dock to build a bowl or raft on which the bottom of the tank group is erected, and which extends radially outwards to form a foundation for the surrounding perforated wall. This is followed by sliding formwork of the vertically extending concrete structures, lowering the pier structure to a location, and lowering it so that it stands on the seabed, as described in Civil Engineering, August 1973, Gerwick Jr., Ben C. and Hogenstad, E. - " Concrete Oil Storage Tank Placed on North Sea Floor".

Two disadvantages of the known construction and method are evident during the attempt to determine the dimensions of the pier upwards for placement in very deep water and storage of a liquid volume of approx. 360,000 m 3. Firstly, the larger moment arm the lateral pressure from the wave forces is directed against in the construction, which increases the risk of sliding and unsafe rocking displacements. Secondly, the greater cost per storage volume unit arising from the longer fabrication time and the increased quantities of reinforced and prestressed concrete required to build the perforated perimeter wall. A protected construction site with deep water and where a pier can be built with a height of approx. 300 m or more, is also rarely available within safe walking distance from the final location.

Even if the storage chambers in a pier according to the known technique, in order to reduce the wave pressure, can be lowered so that the upper parts do not extend above the water surface, the aforementioned difficulties will be applicable together with arbitrary climatic obstacles, such as frequent bad weather, distances from suppliers of construction materials, and costs of maintaining a large workforce using supply ships.

The pier construction according to the invention comprises a vertically elongated body standing on the seabed in deep water, i.e. approx. 90 - 365 m, in one piece with which a storage container or container group occupies positions well below the level in the sea where deep water gravity waves during propagation exert large pressure forces against vertical surfaces with a large span. If the maximum expected amplitude 2a = approx. 30 m and the period of such a large wave is 16 or 17 sec., the lateral wave forces acting on a large storage container with width and height respectively greater than approx. 60 m near sea level be of the order of 10 t/m<2>, but if the top surface of the container is approx. 90 m below the mean water surface, the container may be subjected to forces" not exceeding 1.5 t/m .

The pear shape according to the invention is, in accordance with

the claim's characterizing parts, essentially a very large container or group of containers with end cover plates supported above a rectangular, flat, thick plinth plate of

short, vertical, parallel, load-bearing walls with spaces along the plinth, which are approx. 90 - 120 m long and approx. 60 - 75 m wide. The load-bearing walls extend upwards through the container or group of containers in the form of partitions which accommodate vertical and horizontal loads acting on the container, as well as loads exerted on a system of tubular vertical columns and

horizontal bracing pipes which are upward extensions of the partition walls above the container. The system of vertical columns extends at least to the level of the largest waves and carries on the upper submerged ends a cross wall or bracing system joining the upper edges of two flank walls via curved end portions. The container or group of containers has a shape substantially like a cylindrical, partition-divided body with the cylinder axes horizontal in the width direction of the base. The two protective flank walls are perforated and run separately at a distance of approx. 12 m from the sides of the container above the plinth to a height of approx. 18 m

below the surface of the water, as generally planar plates braced relative to the container and the vertical column group by respectively transverse partitions extending through the container and hollow tubular horizontal beams with vertical spacing. The side walls, which are perforated to a large extent by transverse channels, mainly carry relatively smaller vertical loads.

A system of chambers situated horizontally, parallel to the container axes, and the width dimension of the flank walls, is joined in one piece with and intersected by the vertical column group. These chambers are placed so that the upper sides are approx. 18 m below the mean water level (MVF) and braced in relation to the upper edges of the flank walls using parts of the aforementioned transverse wall.

While the flank walls in the standing pier serve to reduce lateral pressure forces that seek to cause the pier to shift along the seabed or rock about a transverse axis in the shelf, the most important function of these walls is to ensure that the pier body will have very low coupling with wave energy during buckling as a seagoing vessel, i.e. the structure's reflection factor against waves becomes small compared to an unprotected pier, so that minimal wave-induced tensile stresses develop in it regardless of whether the entire pier body floats in the surface layers in which the maximum pressure forces are exerted by waves whose amplitude can be more than half of its draft.

The invention also allows the construction in a relatively short time of a pier with hitherto unattainable storage capacity and vertical height, and it enables the transport as a seagoing vessel of such a pier over large distances with safety and greater speed prior to the final placement. This includes a pir location in which the structure's longest dimension is parallel to the horizontal plane and the container axes are vertical, so that the height dimension during the entire construction phase is smaller than both the length and the width. The exit arrangement in dry dock orients the flank walls towards the sea, and sliding formwork takes place along the greatest possible perimeter length of the formwork so that the floating stage is reached in a small number of "lifts" or steps. The bottom of the container and the bottom of the upper control chamber system are formed by the respective covering end walls which form bowls when initial side wall parts are cast at the edges thereof, so that buoyancy is achieved early during the construction work. The partition walls and the plinth, which have negative buoyancy, are sliding formwork to a height of approx. 9 m, and in connection with integrally joined, form-supported columns, cast with tubular form in a horizontal position together with cross-bracing tubular elements, structural rigidity is achieved at an early stage of construction by this casting in one piece. For the flank walls, buoyancy is provided at this point by closing the ends of the channels.

After anchoring the self-floating, initial pier form to moderately deep water in a protected area, e.g. a fjord, the pier is completed by sliding formwork of all wall parts upwards. The tank group and the control chamber system are ballasted and the pier is braced in the horizontal position in which it was built with the plinth aft and the upper columns directed forward, while the curved top wall and/or the control chamber system serves as a broad bow. During transport, the vessel is relatively immune to large-amplitude transverse waves due to the wave-dissolving effect of the flank walls, which also serve to dampen the bow waves that develop when the vessel moves forward. When the vessel arrives at its final location, the container or group of containers is ballasted and pressurized by means of equipment carried on one edge of the platform and extending from the tubular columns, to impart bottom weight to the vessel, while regulation shames the spaces by needs, de-ballasted to achieve a creating moment for turning the pier body around a transverse axis in its upper part. The pier then floats stably in a vertical position and mainly submerged. Finally, it is lowered to rest on

an advantageous flat area of the seabed. Additional seawater ballasting to fill all tank spaces and the vertical and horizontal columns provides pier anchorage and prevents excessive stress on the tank walls caused by hydrostatic pressure differences between the interior and the sea. Building the final deck structure, installing cranes and drilling rigs and placing supplies ensures adequate pedestal storage pressure. Drilling can take place by utilizing one or more columns in the group that do not cut through container or control chamber systems.

The seabed area preferred for placement of the pier will be an undisturbed, reasonably smooth and level horizontal natural sedimentation of sand, silt or clay of significant thickness overlying a competent rock structure. Core samples will preferably be taken to determine the ability of the layers to bear a distributed vertical load which may be greater than approx. 11 t/m<2>, and to resist a lateral pressure exerted by the wave and current movements of the sea water, which can be greater than 20,320 t. Where there may be bottom-fed volcanic ash that is "grease", e.g. of bentonite character, the site may be unsuitable unless rock ballast embankments are piled up on the plinth flange to stabilize the pier. In general, the ratio of horizontal forces to the submerged weight of the pier will be made sufficiently low that the resultant of the horizontal and vertical weight loads shears the plinth within a prescribed portion of the plinth dimension from its center.

The lower side of the pier base can be conventionally ribbed to develop maximum sediment shear strength.

The invention is explained in more detail below with reference to the schematic figures, of which fig. 1 is a perspective drawing of a pier according to the invention standing on the seabed, fig. 2 is a plan view partially in section showing a series of horizontal sections of the pier of FIG. 1, fig. 3 is a perspective drawing, seen from above, showing the pier platform with equipment for drilling wells and producing oil, fig. 4 is a side view, partly in section, showing the initial preparation of the pier under development in a dry dock by the sea, fig. 5 is a side view of the pier in fig. 4, built up to the self-floating breechable stage and during towing to a next construction site in deeper water, fig. 6 is a view corresponding to the left part of fig. 4 and shows the pier with the opposite orientation to facilitate floating in connection with a large base flange, fig. 7 is an axial section through a channel for the flank wall of FIG. 5, and shows the arrangement of end caps for increasing the initial pier construction's positive buoyancy, fig. 8 is a perspective drawing of the manufacture of a flank wall, a lateral bracing wall and a container side wall, fig. 9 is a plan view which, in an X-ray drawing, shows the contours of the container, the side walls and the base in connection with the placement of pre-tensioning cables, fig. 10 is a horizontal section through the edge of one flank wall in fig. 1 and on a large scale,

fig. 11 is a section in the middle vertical wall of fig. 2 and shows piping in the regulation chamber (line 11 - 11), fig. 12 is a section through a vertical pillar group in fig. 2 and shows passages through the regulation chamber (line 12 - 12), fig. 13 is a side view, partly in section, showing the projection on a vertical plane at the line 13 - 13 in fig. 2, but rotated into the horizontal construction stage to illustrate the casting of a container end wall, FIG. 14 is a section along the line 14 - 14 in fig. 13 and shows an opening into the middle pillar, fig. 15 is a side view of the completed, horizontally floating pier and shows freeboard reduction and forward trimbal loading after arrival at the location, fig. 16 is a further side view of the pier in fig. 15 and' shows ballast regulation to achieve initial self-righting to a stable, vertical floating position, fig. 17 is a side view of the pier in fig. 16 after the transfer of trim ballast to the lower container spaces with an indication of the principal forces affecting the construction, fig. 18 is a side view of part of the pier in fig.

17 after filling all internal spaces with liquid, indicating vertical loads on the base, fig. 19 is a diagram showing horizontal pressure forces acting on vertical surfaces caused by ocean water movements at various depths and corresponding to a top dimensioning wave with amplitude 2a = approx. 30 m, fig. 20 is a side view of another embodiment of the pier according to the invention intended for placement on the seabed at great depth and shows group drilling of oil wells and production from a formation below this, fig. 21 is a diagram for stability analysis of the pier shape according to the invention placed on a seabed of clayey sediment materials, fig. 22 is a corresponding diagram for the pier standing on compacted, hard sand bottom, and fig. 23 is an end view of an alternative construction module for building a flank wall with a large opening ratio.

In fig. 1 shows a pier 10 according to the invention which stands on a seabed 11 and comprises a thick plate plinth 12 with flanges 13 which increase the plinth's length and width. The flanges 13 can have a tapering thickness as shown. In a particular embodiment, the length of the base can be approx. 95 m and the width approx. 65 m with a flange width of 7.5 - 9 m.

The vertical and horizontal loads on the pier are transferred through a system of three vertical walls joined in one piece with the plinth 12 across its width and comprising a central wall 14 with an auxiliary wall 15 at a distance of 25 - 30 m on each side. The walls 14, 15 are massive and cast as reinforced, prestressed concrete structures using a technique which will be explained later. They have a height of approx. 90 m and extends through the interior of a closed, mantle-walled, molded container body 16. The walls 14, 15 have a horizontal dimension in the width direction of the plinth which coincides with the distance between covering end walls 17 which close the container 16 and are molded in one with these. Relatively short lower parts of the walls 14, 15 extend between the base 12 and the bottom section of a convex container side wall 18 rounded with the width of the base, which has axis-parallel curvatures. The walls 14, 15 end at joints with the upper section of the wall 18.

Horizontal forces, directed at the pier structure, are partly transferred to the base through the walls 14, 15 and partly through horizontal bracing walls 19 which vertically intersect the walls 14, 15 at two levels, respectively at 1/3 and 2/3 of container height. The walls 19 extend outside the wall 18 and end as if in a joint with flank walls 20 which extend upwards from the base 12 and past the container 16 to such a height that the upper, inwardly bent end parts 21 of these walls lie at a depth of 18 - 27 m below the mean water level (MVF). The walls 20 are essentially vertical, at least throughout the lower extent, but can slope slightly inward, straight or very slightly curved, at heights above the container 16. The cross-section is in any horizontal plane bounded by straight, parallel lines. Curved upper parts 21 are curved e.g. about horizontal axes parallel to the plinth width. The distance between the container 16 and a wall 20 is 12 - 18 m.

The walls 20 are joined in one piece with a horizontally extending massive top wall 22 between the walls, while the greater part of the vertical extent of the walls 20 is perforated by a large number of transverse channels 23 placed in a regular distribution pattern. The channels 23 are designed so that they constitute effective flow-forming elements through which the water can move freely with low hydraulic pressure differences, so that it continues to move in a jet-shaped manner in an axial direction far inside the walls 20 after outflow from the channel ends. The operation and design of these openings or passages are fully described in the aforementioned US patent and are therefore only briefly discussed here. In order to achieve the lowest resistance to mass transport of seawater through such a perforated wall, the channel diameter and length must both be approx. 0.9 - 1.2 m, and the limits for the channel length are from 0.8 to approx. 3 or 3.5 times the diameter, provided the cross-section is circular. However, the cross-section of the channels can also be elliptical, rectangular or formed as a combination of straight sides with curved corners. The cross-section can be uniform in the axial direction or a slight taper can be used and then preferably towards the interior of the pier. Where the highest opening ratio is to be achieved, the cross-section can be essentially square with round corners and the concrete cross-section reduced between neighboring channels with the constant possibility of placing rebar and prestressing cables. Where the walls 20 will be exposed to the greatest wave stresses in the sea-surface area, e.g. when the pier is braced as a vessel, strength requirements may limit the opening ratio to 40% or less, i.e. the concrete surface area will represent 60% or more of the total wall surface.

The walls 20 serve as wave energy-transforming elements which provide a very small reflection of the waves which normally strike against them or at a certain angle to the normal, when the opening ratio is at least 30%. As is known, can the particle circuit movement in a long-period deep-water gravity wave with an amplitude of 2a = approx. 30 m at the sea surface, is also significant at a depth of 0.4 times the wavelength, as the circuit diameter decreases non-linearly with depth to approx. 1/10 of the surface value at approximately 0.3 times the wavelength, as will be seen from the later-mentioned Fig. 19. The walls 20 are completely effective for converting wave motion around generally horizontal axes in seawater exposed to gravity wave disturbances in the open sea, to only unidirectional motion, as the wave is refracted and the axial jets, guided by the channels, contain kinetic energy which, from the original wave, can quickly dissolve into eddy turbulence that develops heat via viscous friction phenomena.

The channel openings in the outer surface of the walls 20 are preferably curved as parts of a torus 24 (Fig. 7) to improve the efficiency of converting hydraulic overpressure into kinetic flow energy.

The top wall 22 carries an arrangement of columns 25 placed at intervals in a regular distribution pattern. The columns 25 extend through the water surface above the highest wave level which can reach around +18 m, and they carry a deck structure or platform 26 at their upper ends.

Between the top wall 22 and the upper surface of the container 16 runs a number of vertical columns 27 whose lower ends 28 are molded together with the container wall 18, and whose upper parts pass into a regulation chamber 29 located below the wall 22. The walls 14, 15 and respective groups of columns 27 align vertically (fig. 1, 2 and 3) and are arranged with spaces in the width direction of the pier as hollow tubular constructions in groups of seven so that the outermost columns generally align with the container walls 17. The columns 27 can have a larger cross-sectional area in places close to the middle of the pier, and those at the outer end preferably have smaller cross-sectional dimensions and greater length to reduce wave stresses. The total cross-sectional area of the columns 27 must be sufficient for the transmission of all vertical loads originating from the platform 26 with equipment, the wall 22, the chamber 29 and the upper parts of the walls 20, as well as wave pressure directed vertically downwards against the wall 22.

The walls 20 and the columns 27 are joined as a structural unit by means of a system of horizontal tubular beams 30 running in the transverse direction of the pier. The arrangement shown comprises three beam layers each with four beam rows 30, of which the outermost rows have side surfaces 31 flush with the container walls 17 and the others are placed along lines through the third points of the walls 20. The bottom beam layer only extends between the walls 20 and the container 16 at the upper ends of the walls 15, while the top beam layer correspondingly extends between the walls 20 and the bottom of the chamber 29 at the columns 27. The middle beam layer is continuous from one wall 20 to the other.

Further tubular beams 32 extend between the columns 27 in the width direction of the pier, perpendicular to these and the beams 30, and are arranged with a beam layer at the level of the continuous beams 30 and another beam layer at the level of the upper beams 30 at the chamber 29. The columns 2 7 and the beams 30, 32, as mentioned, are all hollow constructions with a cross-sectional shape which causes relatively low tensile stress when exposed to seawater movements. However, rectangular cross-sections with round edges can be used where the manufacturing economy requires it, at the expense of slightly increased tensile forces and the resulting horizontal pressure. The longest cross-sectional dimension should preferably lie in the direction of flow for which the stretch is to be reduced, i.e. that the columns 27 should have a larger cross-sectional dimension in the pier's width direction than in its length direction, and the horizontal cross-section dimension of the beams 30, 32 should be larger than the vertical one. The elements 27, 30, 32 must have a cross-section whose moment of inertia and column length are such that they are structurally safe under combined compression, torsion and bending loads originating from horizontal pressures.

From fig. 1 and 9 it can be seen that the intersecting walls 14, 15, 19 form a group of closed spaces comprising an upper pair 33a, 33b, a middle pair 34a, 34b and a lower pair 35a, 35b, of which the upper and lower pair are partially enclosed by the container wall 18. Closed compartments 36a, 36b, 37a, 37b, 38a, 38b are each surrounded by a segmental portion of the wall 18 and the adjacent wall 15. Each compartment is provided with a system of channels or passages (not shown) which can be welded steel pipes stored in the walls 14, 15 and extending through certain columns 2 7 and 25 to the platform 26. The twelve rooms constitute a very significant storage capacity for liquid materials, e.g. oil, salt water or solutions. Various handling devices can be installed to, by remote control from the platform, place, remove or transfer the materials as desired, and can include pumps and motors of a conventional nature for known use in connection with large volumes of oil and water.

The construction of the regulation chamber 29 partially corresponds to the design of the container 16 by being a divided, closed casing body with a convex rounded outer wall 39, at least one middle vertical partition wall 40 and covering end walls 41. The area of a wall 41 is relatively small in comparison with the area of a container end wall 17, however, the internal volume of the chamber 29 must be sufficiently large for a suitable ballast, as discussed later. The distance between the curved parts 21 of the walls 20 and the chamber wall 39 must be substantial, in order to allow a spread of rays flowing inwards through the upper wall openings 23, e.g. about. 12 m or more.

The wall 40 is preferably used as a support for the larger vertical pipelines used for transporting materials between the platform 26 and the rooms 33a - 38b. From fig. 11 and 12 it can be seen that the wall 40 has tubular passages 42 for the placement of wires through which materials can be passed from a column 27 to a column 25 or vice versa, and a middle extended passage 43 can be arranged for the transport of equipment. At the lower end of the passage 43, a bulkhead is preferably arranged for a lock flap which can isolate the space in a middle column 44. The upper parts 45 of vertical columns 27, which take up positions in rows to the left and right of the middle row in which the column 44 are located, tubular extensions are joined in one piece with convex side wall parts of the chamber 29. These extensions can be completely isolated from the rooms 46, 47 in the chamber 29, or there can be access to them via sluice valves or the like (not shown). Suitable passages 48 can be arranged in the upper part of the wall 39 so that wires or pipes can be led through certain adjacent columns 25 to the rooms in the column parts 45. Especially these rooms in connection with the rooms 46, 4 7 can, as described later, is used for placing powerful pumps with a large volume capacity during jacking and self-righting for ballasting the container 16 and ballast transfer, as substantially large internal parts of the rooms 46, 4 7 will remain empty until the pier is placed on the seabed.

Two additional columns 49 of large dimensions are provided to serve a primary purpose in a pier structure with large liquid storage capacity. The columns 4 9 extend from the upper side of the base 12 to the platform 26 on either side of the container 16 and within the walls 20. Each column 49 forms a protected bore passage which is not connected to any part of the rooms 33a - 38b or the rooms 46, 47. These passages are constructed as rigid, cast pipes of large diameter, suitably reinforced and prestressed, e.g. circumferentially and axially, to protect the drill string during the execution of group well drilling (discussed later) and to protect oil well pipes extending from the seabed to the platform. A large number of boreholes can be drilled from the platform using derricks 50, 51 (Fig. 3) with travel rails 52 which allow the derrick to be positioned over the sector of the bore passage corresponding to the entry position of the borehole through the base 12. Each column 49 is fixed on the outside of the container wall 18 and is joined in one piece with a wall part 21 at the point of penetration. As shown in fig. 2 there is a smaller but useful distance between the walls 20 and the columns 49, so that tensile forces from sea water jets are reduced.

The following part of the description is directed in particular to the construction of an embodiment of a pier according to the invention. It will be understood that the specified dimensions are in no way limiting. For example, the pier can have a standing height of 230 - 24 5 m above the seabed, a length of approx. 100 m and a width of approx. 65 m, and be designed to support a superstructure at a height of at least approx. 18 m above mean water level (MVF). A flanged plinth with a length of approx. 115 m, a width of approx. 85 m and a thickness of 2 - 3 m carries a series of downward-facing anchoring ribs 53.

The initial construction site is chosen primarily with regard to the availability of large quantities of high-quality concrete materials and the availability of a coastal area that enables cheap excavation work and dam construction using local sediments and that is close or reasonably close to deeper water of 50 - 60 m that is protected from deep sea waves. The climatic conditions in the construction area must also allow reasonably long storm-free working periods and the absence of severe cold. A typical construction site (Fig. 4) has an off-shore depth of 9 - 15 m above sediments 54 overlying bedrock or consolidated segments 55, and a smoothly sloping seabed between land and deeper water. The first construction stage is reached when a barrier 56, e.g. a closed sheet pile wall of rolled steel profiles is driven into the sediments 54 to form a perimeter enclosing a length of coastline. Sediment materials are then piled up to form a dam 57, on both sides of the barrier 54 and the seawater is pumped out to dry the construction site. After removal of materials to evenly deepen the formed pool to a level at the base of the dam 57, followed by continuous pumping out of seepage, the actual fabrication begins on a flat bed of crushed stone or gravel 58, smoothed out to provide an even construction site.

A container end wall 17 is cast on a planar mold with a smooth upper surface (not shown) to provide a tight seaworthy surface. At the same time, the base is made with flanges, preferably as a waterproof box construction to create positive buoyancy, and the lower edges of the walls 14, 15, 19, 20 are laid out. When casting the lower edges of the walls 20, the surface of the molds should be polished steel to form the outer edge of the standing wall as a smooth rounded body. The concrete composition should preferably also be selected and the casting conditions regulated so that a dense and durable mass is exposed to seawater moving along the edge 59. After an initial "foundation", a wall 20 is built up in the manner shown in fig. 8, the channels being made as pre-cast sleeves 60 formed in one piece with square flange plates 61 comprising smooth mouths 24 with a diameter of 0.9 - 1.2 m and a distance between the outer sides of the plates 61 of 1.5 - 1, 8 m depending on the chosen wall thickness.

As the inner surfaces of the sleeves 60 are exposed to incessant seawater movements, often with a relatively high flow rate, it is desirable to specify a concrete mixture with a water/cement ratio of approx. 0.4 0, casting in polished steel forms with good vibration to ensure a very dense and smooth surface, absence of rebar, and a greasy mixture to achieve a concrete strength above approx. 55 MPa.

Construction of the walls 20 continues by stacking the flange plates at the specified axis-to-axis distances horizontally and vertically preferably on a square grid in the vertical plane, placement of solid rebars horizontally and vertically between neighboring rows and columns of sleeves 60, placement of pipes 62, 63 to accommodate pretension cables horizontally and vertically, clamping a formwork to close the ends of the shown spaces 64, e.g. using the frame 65 with holding elements 66, 67, and adding a concrete mixture to fill the spaces 64. Concrete with a 28-day strength of at least approx. 41 MPa. The mixture must contain a hardening retarder to ensure that placement of the concrete around the perimeter, a wall length of approx. 360 m, will be carried out within the castability period. A successful combined cure retarder and air entrainer is a sodium lignosulphate. The mixture must be greasy and very sandy with equal parts by weight of fine and coarse aggregate, an aggregate/cement ratio of 4.0 and a water/cement ratio of 0.40. Normal precautions must be taken against too low a temperature (not below 10° C).

At the same time as the walls 20 are erected in "lift", sliding formwork of the unperforated structural walls continues using a perimeter form of which a part 68 is shown in connection with the container wall 18. The extent of the required sliding formwork will be understood from fig. 9 which shows the contour of the concrete construction with horizontal and vertical prestressing cables and pipes 69, 70 placed during the course of the work. It is appropriate to introduce the high-strength bias cables in metal pipes with smooth inner wall surfaces. The exact placement of the biasing elements does not necessarily correspond to fig. 9, which only illustrates the general principles for ensuring that no part of the finished construction will be exposed to tensile loads. In a typical arrangement, bias cables are placed as peripheral parts of arches following the curved shape of the container wall 18 with anchorages in recesses which are open towards the spaces 33a - 35b at the joints with the sides of the walls 14, 15, 19. Further arches with U- bent shape can be placed with the U-part 71 in engagement with a wall 20 and the ends in anchors 72 in recesses in the joints between the wall 14 and the walls 19.

A series of curved, vertical tubes 70 can also be placed in the solid walls as shown in fig. 8. Since the vertical height to which the walls can be built up in dry dock is limited by

the flow draft in an initial stage, and this limited high-

they must be coherent and self-supporting when floating begins,

it may be desirable, when this height is reached, to introduce tensioning cables and carry out the hydraulic jack lifting of the down-

re perimeter part and the vertical cables and then leave inserted pipes for later use in connection with pre-tensioning cables which are tightened during the completion of the construction.

The chamber 29 is built up in a similar way. The initial dry dock stage is characterized by the large positive buoyancy of the container 16 and the chamber 29, while the massive walls 14, 15, 19 and the walls 20 have large negative buoyancy. To improve the situation, a number of the lower layers of flank wall channels are preferably given a positive buoyancy by placing end caps 72 (fig. 7) which at the edge 73 fit tightly into the channel openings 24, provided with a watertight closing flange 74. Such an -schemes are only intended for temporary improvement of operations

ten to the wall height of the principal buoyancy bodies 16, 29

is such that the flank walls can be supported without aids.

The vertical columns 27 are cast using sliding formwork and internal scaffolding and are joined with the other structures in one piece. The bottom layer of columns 25 is cast using a scaffold 75 which stands on the bottom of the dry dock, or each of these columns can be cast in advance and then be cast together with the platform 26 and the top wall 22 respectively. If there is a need for the production of other columns 27 and beams 30, 32, further scaffolding is arranged to support the horizontal extent of these tubular buoyancy bodies.

It is desirable to achieve a certain positive buoyancy for the base 12 in the dry dock stage in order to reduce the water depth required to clear the dock bottom. The base is then molded with relatively thin outer walls 76, 77 with transverse walls (not shown) which may be extensions of the walls 14, 15, to form an open box construction.

If a relatively narrow flange 13 is to be cast, the pier orientation shown in fig. 4, as trouser ring from the top end is more convenient. If a significantly wider flange is to be built, the opposite orientation (fig. 6) is preferred, with the plinth directed seaward, as this requires much less excavation work, and a breast wall 78 can be advantageously used as an aid in the casting of the flange.

The required initial height of the container wall 18 and

the chamber wall 39 can e.g. be 9 - 12 m. At least two and preferably three of the seven layers of vertical columns 27 will be built up joined in one piece with the container 16 and the chamber 29 to help support the walls 20, although these may be brought up at a height which provides a tight freeboard if increased positive buoyancy is not provided for the channels 23. The end of the dry-docking stage is reached by letting in seawater for safe floating of the structure. At this point, the dams 57 are opened sufficiently and a suitable part of the barrier 56 is removed to lead the pier longitudinally out, and the structure is floated to deeper water with the necessary care (Fig. 5).

By using suitable anchoring means, a mooring place (not shown) with a water depth of at least approx. 40 m, where little or no wave movement has been detected, and here the construction work continues in the manner described above until the vertical height of the structure has reached the specified width dimension of the pier.

As the mid-width stage of the floating structure is reached, the columns 49 are produced in conjunction with the wall parts 21, the walls 19, the convex parts of the wall 18 and the base 12. This can be carried out using two principal methods. The tubular sections of the columns 49, whose diameter is approx. 9 m, is cast in advance and floated in the initial pier stage. These sections, which are made with reduced wall thickness and in lengths to extend between and be stored in e.g. two walls 19, are now lifted into place in seats (not shown) in the walls. The thickening of the walls is carried out while the columns are partially floating using internal scaffolding and forms. Alternatively, scaffolding is erected by using the beams 30 and the walls 18, 19 as support for formwork, in which the sections are cast above the water surface. In a similar way, the wall parts 21 and the platform 26 are used to carry formwork for the outer parts of the columns 49.

All the specified hollow, tubular parts of the construction are made with dry internal spaces, so that a substantially positive buoyancy helps to support forward parts of the pier which have negative buoyancy, as long as the chamber 29 does not have sufficient height, i.e. the platform 26, the wall 22 and columns above the water surface. When the chamber wall 39 has reached a given height which can be determined by calculating the displacement and construction unit weights, some water ballasting may be necessary to trim to level.

The production of the upper walls 17 and 41 comprises casting homogeneous, relatively thick plates (ie approx. 0.75 ifcmi) which span distances of approx. 27 - 30 m at a height of 60 m or more above the lower walls (fig. 13). Since, moreover, the finished end walls, especially wall 17, when submerged during the creation of the pier, must withstand substantial hydrostatic pressure that is not balanced by internal liquid pressure, at least for a time until the spaces are filled with liquid, the use of hatches is necessary for removal of internal scaffolding and formwork. Accordingly, the production of the upper edge portions of the walls 14, 15, 18, 19 includes molding of inner perimeter shelves 79 approx. 3 m below the upper limit of these walls (fig. 14).

The scaffolding arrangement comprises beams 80 which may be in the form of steel profiles with the ends resting on the shelves 79 and carrying girders 81 which span distances significantly smaller than the wall-interspace, e.g. about. 9 m. A system of posts 82 supported on the girders 81 and beams 80, supports a formwork 83 with a smooth surface. After casting and hardening of the walls 17, the scaffolding and formwork can be removed along the shelves 79 and led out via the uppermost columns 27 through openings 84 formed at the ends of these at the junction with the container wall 18 on both sides of the wall 14.

The arrangement of the shelves 79 and other shelves 85, 86 in an intermediate and a lower level facilitates the setting up of scaffolding during sliding formwork, finishing of the inner container surfaces as desired, and anchoring of tension cables placed as vertical columns. Certain pipes can e.g. placed to form a U-loop embedded in the upper wall 17 and with the open ends accessible in recesses 87 at the lower side of a joint between a shelf 86 and the wall 14. It must be noted that the radius of curvature in the U - the bends on pipes containing pre-tensioning cables should be approx. 1.2 m.

After removal of all temporary construction material and waste, any opening 84 is hermetically sealed by molding. Channels 88 with a large cross-section, arranged in a wall 14 or 15

in sufficient number to enable liquid transfer at a high volume rate, standing in connection with steel pipelines extending through the columns 27 to the surface or chamber 29, constitute isolated and protected means for the transport of substantially polluting liquid >. e.g. oil. All pumps and motors as well as control and power lines (not shown) are placed in the container 16 and the chamber 29 prior to casting the walls 17 and 41, and all jet pumps operated via hydraulic lines from the platform are also installed. The rooms in all the prestressing pipes and anchoring rings are injection molded with concrete.

When completed, the structure has a relatively large freeboard. This can, by pumping in a sufficient volume of seawater into selected rooms 33a - 38b, be reduced to approx. 12 - 15 m, or suitable to withstand any likely wave height that may be encountered during breeching to a distant location. Although the provided vessel has good stability when rolling without ballast, filling the lower parts of the container space will further improve the lateral stability around a longitudinal axis. Suitable trimming ballast can also be placed in the lower parts of the chambers 4 6,47.

At any stage of construction after sufficient increase of the positive buoyancy from the container 16 and the chamber 29, the caps 72 can be removed and salvaged, e.g. by divers, so that seawater is admitted and displaces the trapped air. Preferably, these hoods should have a slight positive buoyancy to facilitate salvage.

When the completion of the platform 26 approaches, it can e.g. on an extension 89 of a middle upper pillar 25, a necessary superstructure is mounted for control with bulging, ballasting, trimming, self-righting and lowering of the pier after it leaves the last construction area. Such equipment can be arranged with a crane 90, including crew quarters, supplies, power installations and means of communication etc. placed in a watertight cubicle 91.

The seaworthiness of the large pier structure in a horizontal position has been shown to be excellent in model studies, carried out with equivalent waves of large amplitude. When handing over the completed pier over a stretch of sea which may be more than approx. 1,800 km, the journey can take weeks to complete, although a good cruising speed of 2 - 3 knots is possible for the construction, and in this time frame all imaginable sea and weather conditions can occur. The relatively fragile structure is able to preserve its structural integrity despite the application of periodic lateral forces of large magnitude, e.g. about. 51,000 t if a large wave runs with the broad side towards the length of the vessel. The entire submerged part of the construction, e.g. 45 - 50 m, lies in the sea surface layers in which the particle circuit movement and wave pressure forces are close to the maximum. For a long-period wave (12 - 17 sec), which can propagate with an amplitude that can exceed 23 m, the extended length of the pier represents a half-wavelength or more and consequently the impact of the wave energy on the structure must be expected to be severe. Here, reference can be made to the curve in fig. 19 which shows forces exerted against a., reflecting vertical surface of large dimensions with an amplitude 2a = approx. 30 m. The walls 20 are not only designed and placed to serve as a "hull" for the pier as a seagoing vessel, but to equalize incident waves and limit to the smallest possible the reflection of wave energy that would give rise to an increase in amplitude. Whereas heretofore stationary breakwaters have been constructed to perform a similar function in relation to a rear imperforate wall, the walls 20 constitute a movable, floating breakwater which encloses the container 16 and the chamber 29 behind two essentially parallel, perforated outer walls. Several interesting properties have been found for the combination of these two walls with an extended, rigid, internal non-perforated construction during buckling. Surprisingly, a much reduced horizontal draft is experienced in varying sea conditions when the wall 22 faces forward as a broad pile. The normal bow wave turns out to have a significantly reduced amplitude. A finely patterned turbulence develops in the seawater at the sides of the walls 20 with a tendency to dissolve and distribute the energy from incident waves. The vessel has an extremely long pitching period and high stability around any horizontal axis.

On arrival at the final destination, the pier's freeboard is intentionally reduced significantly as shown in fig. 15. This figure shows in an X-ray drawing the vertical projection areas of water ballast volumes pumped into rooms 34a - 35b and 37a - 38b.

As an initial step to the construction stage, studies are carried out to determine the distribution of displacements and masses required to ensure that the group of columns 25 can be lifted out of the sea without risk with the least possible root stress at the joints with the wall 22. As this group includes columns with a hollow mantle shape that are relatively slender, e.g. about. 3 - 4.5 m in diameter, and has a length of more than approx. 35 m, it will be understood that the moments around a cross-section at the root of a single column and its share of the platform would give rise to unreasonable compression stresses if the column were retained as a cantilever beam. If, however, the righting rotation is carried out around an axis relatively close to the wall 22 and sufficiently below the water surface, so that the top row of columns is inclined relative to this during the righting and the subsequent rows are further inclined so that the majority of the rows come above the water surface at angles of less than 45° with the vertical plane, a safe load distribution is achieved between the individual columns despite small wall thicknesses at the root joints. The large beam depth represented by the bore passages 49, fixed at the upper ends in the platform 25 which joins and binds the upper ends of the columns, also provides a strong support for the group with low deflection.

By virtue of the positive buoyancy contributions from the bore passages above the wall 22 and the small contributions from the submerged columns, these can be relatively slim and thin-walled.

Fig. 15 shows the various gravity loads, i.e. the trim ballast WT, the structural dry weight WS and the container ballast WB, in equilibrium with the composite buoyancy force B associated with the submerged pier. It can be seen that the largest individual weight load WS acts in the center of mass CMs which is located in the container 16 just to the left of the upper wall 19, and furthermore that the relatively largest part of the chamber 29 is occupied by the trim ballast

WT.

In order to bring the pier into the initial self-righting state (Fig. 16), and also to ensure that the righting rotation will take place around a horizontal axis inside the chamber 29 close to the wall 22, the bottom part of the pier is first lowered, so that the container 16 is brought almost completely below the surface of the water. For example, magpies

the mean water surface (MVF) the wall 17 along a line m which lies in

a distance "X" from the direction of force of the buoyancy vector B,

and the wall 17 lies obliquely at an angle 0 with the horizontal plane. This is achieved by increasing the ballast in the spaces 34a, 34b and 37a, 37b to the maximum, which adds mass relatively close to CMg, and by further ballasting the spaces 35a, 35b and 38a, 38b. The vector B shifts slightly to the right and acts in a buoyancy point CB which is slightly higher than in fig. 15 and closer to CMg in the horizontal direction. The pier is stable with the composite center of mass CM located vertically below CB and the total mass vector EW equal to the buoyancy vector B. In this condition, the top row of columns 25 and the extension 89 are raised above the water surface. Any further ballasting of spaces 35a, 35b and 38a, 38b will not add significant buoyancy, but will reduce the distance "X" tending to increase the angle θ and shift the vector £W to the right relative to CB.

At a critical time, when the ballast increases in the lower compartments of the container 16, an irreversible loss of horizontal stability occurs as the point CM moves towards the base 12 and the creating moment arm grows. Ideally, a small mass of ballast should be transferred through the chamber 29 to the lower container spaces to reduce the WT. This transfer can be carried out without the use of pumps, just by connecting the WT to the WB, e.g. by means of suitable ports or valves placed at the junction between a column 27 and the wall 39. If the flow cross-section is sufficient, e.g. about. 2 m 2 or more, the hydraulic pressure, caused by the plinth end descending in relation to the chamber 29, ensures a rapid transfer. When this is finished,' e.g. within an hour or less, a steadily increasing rotational speed is applied to the pier, the rotation being greatly dampened due to the large cross-section of the container 16.

At the end of a 90° turn to a vertical position, the pier floats as shown in fig. 17 with the columns 25 largely or completely clear of the water, and the container 16 is more than half-filled with seawater. The pier is very stable in this condition as the total buoyancy vector B acts at the point CB which is displaced vertically and above the mass point CMg of the dry structure and the composite center of mass CM for W. In this condition the pier can be banked as shown with a tugboat 92 with a line 93 attached to the passage 49 on one side of the pier, while another line 94 may be attached to the other passage 49 or other pillars for the purpose of positioning the pier accurately or moving it a certain distance during jacking. It is clear that the majority of the container 16, although under a compressive stress due to immersion at a depth of approx. 90 - 100 m, to some extent has a compensating internal hydrostatic pressure due to the water ballast.

The operation to finally increase the weight of the pier so that it sinks to the seabed 11 is merely a continuation of the initial ballasting carried out either from a vessel alongside (not shown) or by means of equipment in the building 91. The large mass of the ballasted piers, e.g. 500 - 600 thousand tonnes, ensures a slow lowering which is further dampened by the water's cushioning effect under the plinth, as the distance to the seabed is reduced to around one metre.

By only having to sink approx. 18 m, the pier will not be exposed to impacts for the reasons mentioned. Any final kinetic energy will help to cause the ribs 53 to cut into the bottom sediments to prevent the pier from shifting laterally under the influence of local ocean currents. However, it is desirable to quickly increase the size of the vector (V7 + W - B) in the ballasted mass-

so

point CMg. Filling all tubular bodies, i.e. the passages 49, the columns 27, and the beams 30, 32 increases the gravity load.

In fig. 20 shows a pier 100 determined for placement in such a water depth that a maximum dimensioning wave with a period of 17 sec only has a wavelength that is a fraction greater than the depth. The pier 100 generally corresponds to the described pier 100 except that the width (along the container axis) may be greater, and the container 116 has sixteen arches compared to ten arches on the container 16 and with significantly increased storage capacity, e.g. 240,000 - 360,000 m<3>. The pier 100 is also relatively higher in relation to the length dimension. The lower parts of the flank walls 120 are placed with a slightly greater distance from each other and have an increased inward slope of the upper parts, while the upper, outer spaces in the container 116 are designed with a reduced dimension in the longitudinal direction of the pier to provide a beam-distributing clearance distance to the perforated walls 12 0. Drill passages 149 extend through the walls 120 at a shallower depth and upwardly at a greater horizontal distance from each other". The structure can support a multi-story platform 126, with approximately the same clearance above the mean water surface (MVF) as the platform 26.

As shown, a substantial number of wells 96 can be placed

in a group of boreholes which are carried out from the passages 14 9 under the base 12, which holes in a known manner spread out in depth to reach areas with a producing reservoir rock 97.

Figs 21 and 22 show the horizontal and vertical forces acting on the pier 100, represented by vector arrows EF^ and W, and the reactions of the seabed sediments in relation to the base 12.

To first consider the more uncertain condition in which the resistance to downward pressure and horizontal loads can be provided by sediments with a certain plasticity and limited bearing capacity, it is on fig. 21 shows a permanent clay or clayey mud bed 98 as a support for the plinth 12, through which the vector resultant R acts at an angle 0 with the vertical line intersecting the sediment surface at a distance "e" from the center of the plinth dimension "b". The shear strength of test samples of such sediments obtained under conditions which cause the least possible disturbance of the material cohesion generally provides a less reliable indication of the behavior of a large section of the seabed to significant, periodically varying forces.

Since the resultant R is directed eccentrically, the distribution of the sediment reaction's vertical components p.^ - pn will be uneven. In the example shown, the vertical shear force diagram will be trapezoidal, i.e. pn will be the larger unit reaction force at the edge of the base 12 which is closest to the intersection of the resultant. Calculation of the magnitude of such a reaction force is useful, as an arbitrary maximum vertical shear force can be applied. The relationship between pn and p.^ can be found by, for the aforementioned shear force diagram in which the composite vertical reaction F'v acts through the centroid, setting up the equation:

where <=> ~ 2 e •

The permissible eccentricity for a rectangular bearing plate

is equal to <b>/g-

Next, to consider the resistance to sliding, an arbitrary coefficient of frictional resistance can be derived from the sample shear strength, so that

frictional resistance = plinth area x C,

where C = (it + 2)S.

Values of the shear strength for saturated clay sediment can be below approx. 24 kPa.

The safety factor for sliding is determined as

In connection with a compacted hard sand sediment, the classic analysis involves determination of the friction angle. For example, this angle can be 31° between sand and a flat concrete slab, i.e. if the slab rested on a sand surface below this angle with the horizontal plane, the expected movement would begin if the angle were increased somewhat.

The slope 0 of the resultant is calculated, so that

Assuming that the ratio is 0.18 (0 = 10°12')r, the safety factor against sliding is found to be

In general, a safety factor of 2.0 is sufficient.

The above analyzes are shown to illustrate the usability in current conditions at a location for the pier. Poor seabed materials may require substantially expanded plinth plate supports as well as rib and beam reinforcement of the flanges. To reduce the size of the EFH against the flank walls, a higher perforation ratio may be necessary, e.g. up to 40 - 45%, to reduce reflection and consequently pressure forces from an incident wave to a minimum. A design of a modular construction unit for producing the flank walls with improved perforation ratio has, as shown in fig. 23, a channel-shaped cross-section bounded by straight line parts of a square and arc-shaped joining portions. For a given axis-to-axis placement pattern, such a unit does not allow the wall to carry as much load as when using units that provide a smaller perforation ratio due to the smaller concrete content, unless the axial length is increased accordingly.

The preceding description includes the design and manufacture of new piers with large dimensions designed to form stable drilling and production platforms with significant deck area and storage capacity for deep-water drilling. The invention can be practiced in connection with constructions for even greater depths and many other applications than oil production, e.g. underground works such as potassium carbonate mines, coal mining and/or

- gasification etc.

In all the forms in which the piers according to the invention can be realized, the construction's components comprise a deeply submerged, dense container body, shaped with more or less equal width, length and height dimensions and a cross-section arc-shaped side wall configuration, placed in the construction in such a way that the lateral pressure forces from a dimensioning wave of maximum amplitude is greatly reduced due to the depth location and the protective effect of vertical flank walls from two opposite directions. When manufacturing such constructions, the economy is very significant when building upwards under component taping, binding in formwork, and sliding formwork along very extended perimeters. When buckling the construction in a horizontal position, extremely stable and seaworthy vessel behavior is observed. At the final location, the only part of the structure that is vulnerable to the weather and collision with ships is the pillar group that rises from a relatively flat top wall approx. 18 m below mean water level to a height well above the highest wave extent. However, the new construction with a forest of uniformly spaced columns rising above the top wall is singularly effective in ensuring in the event of a collision that only a small number are damaged, and replacement can be made by buckling a pre-cast unit for integrated join instead of the original one.

Although the total projection area of the column group is substantial, the horizontal pressure from an incident wave of large amplitude is low due to low coupling with the structure. The column array serves as an excellent means of converting the energy in deep-water gravity waves into eddy motion and ultimately changing kinetic energy into heat. This will be understood by considering that in connection with waves propagating perpendicular to the plane of the flank walls, with the crest of the wave at the front row of columns, the orbits of seawater particles are momentarily horizontal with the average velocity of the water flowing through the vertically elongated channels between neighboring columns somewhat greater than the circulation speed in open sea that can be calculated for a given wave amplitude. A very low reflection coefficient due to the column diameter will increase the amplitude somewhat and thus the particle velocity. According to classical hydrodynamics, however, the flow pattern in a liquid is very unstable behind a cylindrical obstacle with an axis perpendicular to the direction of flow, as the cooling water streak in a flow that does not persist for a long time is characterized by very regular individual vortices which, formed on both

.sides, moves with the liquid in two parallel rows, in which the rotations of output paired vortices are opposite and these are displaced along the cooling water in a symmetrical arrangement. The eddies are separated by laterally undulating streamlines and they do not mix with the outer flow and dissolve by viscous friction only

over a longer period of time. Moreover, the eddies move in the direction of flow at a speed which is less than the seawater's flow speed in relation to the fixed columns. It is clear that the relatively long-lasting axis-vertical vortices in the Karman strips behind each column represent a significant transformation of wave energy, in effect analogous to the effect of the flank walls. It will be seen from fig. 1 and 3 that regardless of the direction of wave propagation towards the pirenj, the wave energy can only penetrate briefly into the group of columns, and consequently a calculation of the horizontal pressure only needs to take into account the outer columns of a group exposed to incident waves. This conversion process results in a relatively low stretch, which can be verified by classical calculation methods.

Claims (31)

1. Platform for placement at an offshore location, comprising a plinth plate (12), a tower-like structure connected to the plinth plate and extending vertically upward therefrom, and a deck structure (26) carried above the sea surface by the tower-like structure , as this is provided with perforated outer boundary walls to reduce the impact of the waves, characterized in that the tower-like construction comprises a number of mutually separated vertical first support elements (14, 15) connected to the upper surface of the plinth plate (12) as in a distance above the base plate carries a container body (16) and extends through this, and which is connected at the container body's upper limiting surface to a number of mutually separated, vertical, other support elements (27) which extend upwards to near the sea surface, two vertical, with continuous channels (23) far-reaching perforated flank walls (20) which with their lower parts are connected to the base plate and arranged on each s in the side of the first support elements (14, 15), and which extends for some distance from the side walls (18) of the container body (16) to near the sea surface, a top wall (22) which connects the upper parts of the flank walls (20) and the upper parts of the second support elements (2 7), a number of third support elements (25) which are connected with their lower ends to the top wall (22) and extend upwards to above the highest wave level, which elements (25) with its upper ends are connected to and carry a deck construction (26), as well as a closed chamber (29) which is produced in one piece with the top wall (22). 2. Platform according to claim 1, characterized in that it is approximately rectangular in horizontal cross section. 3. Platform according to claim 2, characterized in that the first support elements (14, 15) are parallel walls that extend in the width direction of the platform's horizontal, approximately rectangular cross-section. 4. Platform according to claims 1-3, characterized in that the other supporting elements are a number of tubular columns (2 7) arranged in parallel rows. 5. Platform according to claims 1-4, characterized in that the third support elements are a number of slender tubular columns (25). 6. Platform according to claims 1-5, characterized in that it further comprises a number of rows of first stiffening beams (32) which extend parallel to the horizontal extent of the side walls (20) between the other supporting elements (27), and which are perpendicular to and is associated with these. 7. Platform according to claims 1-6, characterized in that it comprises a number of horizontal bracing walls (19) and a number of rows of horizontal, other bracing beams (30) which extend between the flank walls (20) and are connected to these, the other stiffening beams (30) are made in one piece with the other support elements (27) at the joints with the first stiffening beams (32). 8. Platform according to claims 1-7, characterized in that the container body (16) has parallel, planar end walls (17) and a convex looped side wall (18) produced in one piece with the end walls, which together with the first support elements (14, 15) form spaces (33a, b - 38a, b), the side wall (18) being cylindrical with generatrices that are perpendicular to the end walls (17) and the side wall (18), and the first support elements (14, 15) are intersected by two horizontal bracing walls (19). 9. Platform according to claims 1-8, characterized in that the closed chamber (29) is divided into two rooms by a vertical wall (40) which extends in line with a wall (14) which forms a middle element of the first supports -elements, which vertical wall (40) is connected to the tubular, other support elements (27) which are flush with this wall. 10. Platform according to claim 9, characterized in that the closed chamber (29) is cut through by rows of the tubular, other support elements (27) which are arranged at some distance from and on each side of the middle row of other supports -elements (27). 11. Platform according to claim 10, characterized in that the slender tubular third support elements (25) are arranged next to each other and in a row one after the other on the upper surface of at least part of the top wall (22), while at least one central row of these support elements (25) has the internal spaces in connection with vertical other support elements (27) in the middle row via passages (42) in the vertical wall (40), and at least certain third support elements (25) in other rows have the internal spaces standing in connection with the extensions of the other support elements (27) which cut through the closed chamber (29). 12. Platform according to claim 10, characterized in that the closed chamber (29) has a convex looped side wall (39) and vertical planar end walls (41), and the flank walls (20) have inwardly bent upper end parts (21) which merge into the top wall ( 22) and is at a horizontal distance from the chamber side wall. 13. Platform according to claim 12, characterized in that the tubular second support elements (27) have a horizontal cross-section whose longest dimension is parallel to the flank walls (20). 14. Platform according to claim 12, characterized in that the slender tubular third support elements (25) which extend upwards above the water surface are cylindrical or have a slightly decreasing cross-section upwards. 15. Platform according to one of claims 1-14, characterized by two tubular columns (4 9) which at the lower ends are made in one piece with the base plate (12) and extend on each side of the container body (16) between the side wall (18 ) and the adjacent flank wall (20), cutting through the deck (26) and the top wall (22) through the convexly inwardly bent upper end part (21) which is made in one piece with the upper part of the flank wall (20), which columns (49) form unobstructed passages with essentially the same extent as the height of the platform. 16. Platform according to claim 15, characterized in that the two flank walls (20) each comprise a lower vertical part and an upper part, which slopes slightly in relation to the vertical plane and at the upper end part (21) merges into the top wall (22), which upper inclined wall parts are each cut through by a column (49). 17. Platform according to claim 1, characterized in that the channels (23) have inner surfaces terminating in end openings (24) shaped as convexly rounded surfaces smoothly joined to the outer surfaces of the flank walls (20). 18. Platform according to claim 1, characterized in that the vertical edges (59) of the flank walls (20) are rounded convex in any horizontal plane. 19. Platform according to claims 1, 6, 7 and 12, characterized in that layers of the other stiffening beams (30) cut through rows of column-shaped other support elements (27) in different horizontal planes with an upper layer cutting through the closed chamber (29) almost tangentially in relation to the lower part of this chamber's (29) side wall (39), and with a bottom layer cutting through the container body (16) almost tangentially in relation to the upper part of this container body's side wall (18). 20. Platform according to claim 6, 7 or 19, character r i s e r t in that the other bracing beams (30) cut through the rows of column-shaped other support elements (27) which extend upwards from the end walls (17) of the container body (16) and at least one row of internal, column-shaped other support elements (27). 21. Platform according to claim 20, characterized in that there are a different number of column-shaped second support elements (27) in the rows that rise from the upper end of the first support elements (14, 15), and two other stiffening beams ( 30) connects the flank walls (20) with each inner row of column-shaped, other supporting elements (27). 22. Platform according to claim 9 or 12, characterized in that a number of column-shaped second support elements (27) rise from the upper edge of the middle of the first support elements (14) and end with their upper ends in the lower part of the side wall (39), which support elements are made in one piece with the vertical wall (40) that extends through the closed chamber (29) in one piece with the side wall (39) and the end wall (41) of the closed chamber (29) ). 23. Platform according to claim 22, characterized in that the vertical wall (40) contains passages (42, 43) which are connected to the inner spaces in the second and third column-shaped support elements (25, 27). 24. Platform according to claim 11, characterized in that internal spaces in the second and third support elements are in connection with the spaces (46, 47) of the closed chamber (29) via channels provided with suitable closing means. 25. Platform according to one of claims 1-24, characterized in that the base plate (12) is a massive body which is formed in one piece and has lateral flanges (13) which extend outside the flank walls (20) and the flat end walls (17) respectively ). 26. Platform according to claim 25, characterized in that the flanges (13) have lower surfaces flush with the lower side of the plinth plate (12), and the thickness of the flanges decreases in the direction from the platform. 27. Platform according to claim 26, characterized in that the flanges (13) have inclined upper surfaces and extend beyond the end and side walls (17, 20). 28. Platform according to claim 1, characterized in that the lower side of the base plate (12) is designed with a pattern of downward-facing ribs (53) which intersect. 29. Platform according to claims 1, 3 and 8, characterized in that the wall-shaped first support elements (14, 15) comprise a middle wall (14) and two outer walls (15), and the loop edges of the side walls (18) are made in one piece with the outer of the wall-shaped, first support elements (15) at the joints between these and the bracing walls (19). 30. Platform according to one of claims 8-28, characterized in that the first support elements (14, 15) are designed with internal passages (88) which are in connection with internal spaces delimited by the end, side and stiffening walls (17 , 18, 19) and the first support elements (14, 15), and the internal passages (88) are connected to internal spaces in the columnar second support elements (27). 31. Platform according to claims 24 and 30, characterized in that internal spaces in the closed chamber (29) and the container body (16) are interconnected by means of a wiring system which includes the internal spaces in the first and second support elements ( 14, 15, 27) and channels and internal passages (88) in these.