IE44972B1 - Iprovements in deep water platforms for marine work - Google Patents

Abstract

1523096 Offshore platforms STRABAG BAU AG 17 June 1977 [19 June 1976] 25508/77 Heading E1H A gravity offshore platform. Fig. 1, has at least one vertical spur 24 which projects below a body 17 and is arranged to be sealingly and axially displaced in a guide shaft 31, Fig. 3, which is sealable at its upper end and forms with the spur a pressure chamber 59 having an adjustable pressure limiting valve. When the platform is being lowered, the chambers 59 are filled with water which is released at a predetermined pressure as the spurs penetrate the, sea bed to prevent sideways displacement of the platform during placement.

Description

The present invention relates to a platform, in particular for marine work, having a foundation body which is positioned on the sea bed, and at least one vertical spur directed towards the sea bed and projecting beyond the base surface of the foundation body.

The invention also relates to a process for the assembly of such a platform to be employed in the sea.

On lowering large-area foundation bodies, such as caissons, cell foundations for building structures, or drilling islands, there occur in the water below the base surface of the foundation body during lowering considerable horizontal flows which in particular directly over the sea bed assume high velocities and exert on the foundation body considerable horizontal forces which dis place them uncontrollably laterally directly prior to and during placing the structure on the sea bed. It is therefore difficult to place the foundation body on the sea bed in the exact position desired. Additionally, there is a risk of damage to sealing aprons arranged under the base face of the foundation body and reaching to a lower level, which said sealing aprons are intended to oppose erosion of the sea bed located under the found· ation body.

Xn order to decelerate the horizontal movements of the base body shortly before it is placed on the sea bed, it has already been proposed to secure to the foundation member , vertical spurs which project beyond its base surface and which penetrate into the sea bed before the base surface of the foundation body, with its sealing aprons, reaches the sea bed and which are intended to prevent lateral displacement of the foundation body in the last stage of the lowering process. However, with this arrangement it is difficult to determine precisely in advance the spur resistance and the generated surface friction of the spurs (of vzhich there are generally several) in the sea bed and it may happen that the spur resistance and the generated surface friction become so large that the spurs penetrate no further into the sea bed and the entire underneath drive for the foundation body is transmitted to the spurs which thereby suffer rupture and may damage the foundation body at the location of securing of the spurs. Furthermore , it may happen that one or more spurs impinge on pieces of rock or denser sea bed layers than other spurs arranged under the foundation body and therefore penetrate into che sea bottom to a lesser depth or less rapidly. The foundation body may then take up an inclined position relative to the sea bed and considerable force is required to straighten it up again. In some eases once the foundation body has tilted to one side , it is impossible to straighten.

It is an object of the present invention to obviate these difficulties and to so design the foundation body that the vertical loading attributed to the spurs is not exceeded at any phase of the lowering process and that inclined positioning of the foundation body on being set down on the sea bed is prevented. It is a further object of the invention to provide a process for the assembly of such a platform which can be especially readily effected with the aid of the spurs.

According to the present invention, there is provided a platform for marine work,having a foundation body which is positioned on the sea bed and at least one vertical spur directed towards the sea bed and projecting beyond the base face of the foundation body, wherein the spur is arranged to be sealingly and axially displaceable in a guiding shaft adapted to its cross-section, which said guiding shaft is sealable at its upper end and forms with the spur a pressure chamber having an adjustable pressure limiting valve.

This design has the advantage that the spur is able to slide downwardly in the guiding shaft, as soon as the vertical forces effective in it after setting down on the sea bed reach a pre-determined maximum permissible dimension during lowering. The upper end of the spur is placed in the manner of a piston in a cylinder in which there is enclosed a pressure medium able to escape by a pressure limiting valve as soon as the desired pressure in the pressure chamber is exceeded. Since this pressure is adjustable, the penetration depth of the spurs can be adapted to the actual nature of the sea bed. Additionally, it is possible to completely relieve the spurs of pressure in the guiding shaft, so as to cause the underneath buoyancy to act completely on the foundation body alone as soon as the spurs have exerted their stabilizing effect and have fulfilled their purpose and the foundation body is to penetrate, with its sealing skirts, fully into the sea bed and be set down closely on the sea bed with its base surface, the foundation body then being guided at the spurs which are fast in the sea bed.

As soon as the spurs have penetrated into the sea bed and the earth resistance has been activated, the spurs are also able to take up vertical loads. If a plurality of spurs is arranged, it is then possible to lower the foundation body in the last phase of the lowering process guided at the spurs horizontally to the sea bed or to align it at the said spurs horizontally over the sea bed.

The pressure limiting valve can be installed in a line which connects the pressure chamber with a pressure liquid container. Furthermore, the pressure limiting valve is expediently also designed as a shut-off valve, a filling and discharging valve being connected in parallel with it. This design has the advantage that the pressure liquid, expediently water, present in the pressure chamber can be pumped back into the pressure chamber when the foundation body is lifted by the spurs inserted into the sea bed, for example in order to align it or to release its base surface from the sea bed.

The spur and its guiding shaft may have different 10 cross-sections. Expediently, both are cylindrical and are designed to be hemispherical, at their upper ends, since this form can be manufactured simply, permits ready assembly and is best adapted to the pressure stressings occurring.

The guiding shaft is expediently sealed by a hemispherical cover which is secured to be pressure-tight and releasable at the guiding shaft and bears at a support structure mounted on the foundation body. The upper, hemispherical end of the spur fits into this hemp20 spherical cover, so that the latter can be retracted completely into the guiding shaft. Since th® cover is removable, it is possible to introduce the spur subsequently, i.e, after completion of the foundation body in the dock and after the same has been launched on to the water, from above into the guiding shaft and only then to seal the guiding shaft at its upper side. This is important in the case if foundation bodies of large structure, in which no supplementary space can be kept free in the dock below the base surface. The wall and/or the hemi30 spherical cover of the guiding shaft may be made from concrete or from steel. It is also possible to manufacture the wall of the guiding shaft within the fundamental body from concrete or pre-stressed concrete and to line it or enclose it with a steel tube.

Provided at the inner face of the wall of the guiding shaft for the spur is at least one annular chamber in - 6 which is locate^ a packing or sealing ring of resilient material which is pressed against the outer periphery of the spur by a pressure hose arranged at its outer side and adapted to be inflated by pressure medium. Such a packing, of which preferably a plurality is arranged in various chambers in the axial direction of the guiding shaft, one after the other in the manner of a labyrinth packing, makes satisfactory sealing of the spur in the guiding shaft possible and at the same time permits axial movement thereof. With this arrangement, the application pressure of the packings can be varied and adapted to the pressure existing in the pressure chamber of the guiding shaft.

For transmitting the horizontal forces from the spur to the foundation body and vice versa, there are preferably arranged at the inner face of the wall of the guiding shaft, a plurality of guiding and support rings which guide the spur and hold it in spaced relationship relative to the wall of the guiding shaft.

The spur itself may have at its upper end a carrier ring laid about its external periphery and co-operating with the guiding and support rings. With this carrier ring, the spur is retained in its lowest position on lowering it. The carrier ring simultaneously limits the structural height of the foundation body if the latter is raised at the spur inserted into the sea bed in order to align it or to lift it off from the sea bed.

The guiding shaft is so long that it is able to take up the spur at least to such an extent that the lower end thereof does not project above the base face, of the foundation body. This design has the advantage that due to the arrangement of the spurs no supplementary floating depth is required if the foundation body is to be floated to the place of use in relatively shallow water.

In order to facilitate introduction of the spur after manufacture of the foundation body and to be able to subsequently draw it completely into the guiding shaft, the spur is preferably suspended from traction means which extend sealingly through the covpr of the guiding shaft and a! witicl: llfiing gear engaged.

If Ifi tsBpee’ially expedient j! the spur is constituted by a tube which is open at the lower end and the interior of which is connected with, the external water by a pressure compensating line. This design has the advantage that the point resistance of the spur on penetrating into the sea bed is very small. Simultaneously, due to the pressure compensating line after penetration of the spur into the sea bed there is prevented any build up in the tube of water pressure which would make penetration of the spur difficult.

In order not to load the foundation body during transport and lowering with supplementary ballast, the spur tubes have an air-filled buoyancy chamber.

The above-mentioned pressure compensation line is preferably constituted, by a tube which extends through the pressure chamber of the guiding shaft and optionally the buoyancy chamber of the spur and is slidingly sealed at the cover of the guiding shaft and which serves simultaneously as traction means for the spur, and which the lifting tackle engages. Then, at the cover of the guiding shaft, the pressure compensation tube can be sealed with a stuffing box. Such a structure is especially simple and facilitates lowering and pulling up of the spur tube in the shaft and simultaneously pressure compensation in the spur tube.

The spurs which are arranged to be displaceable in the foundation body can also serve as guideways for the foundation body on lowering in shallow waters. Additionally, they permit floating-over of the foundation body with, other structures, fcr example with a drilling island platform to be secured at the foundation body, since the spurs during the floating-over process non-displaceably retain the foundation body at the bottom of shallow water.

These advantages can be made use of in a method for the assembly of a large platform to be used at sea and which has provided at its base surface a foundation body - 8 adapted to be set down on the sea bed and having spurs adapted to be vertically pushed in, and having support legs secured thereto and a buoyant deck arranged to be vertically displaceable at the support legs. With this arrangement, the procedure is such that initially the foundation body,'with its spurs retained to be displaceable in load-dependent manner is set down on the sea bed in shallow water and after penetration of the spurs into the sea bed is guided at the spurs and lowered to such an extent that the surface of the foundation body is under the water surface at least by the amount of the immersion depth of the buoyant deck and that then the deck is floated over the foundation body, and after raising of the foundation body to the water surface is secured to the support legs.

This method has the advantage that the foundation body manufactured in the dock can be launched on the water alone, without support legs and deck, after manufacture thereof and in the vicinity of the shipyard which manufactured the deck and optionally also the support decks lowered in shallow water to below the water surface. With this arrangement, the spurs are enabled to reliably guide the foundation body during lowering if the foundation body on being completely immersed in the water passes into an unstable floating position. Thereupon, the deck can be floated in its entirety over the foundation body and secured to the support legs subsequently provided with the foundation body. However, it is especially expedient to secure the support legs to the foundation body prior to floating-in of the deck and to float-in the deck in a plurality of buoyant portions subsequently between the upwardly projecting support legs over the foundation body and to assemble it thereon. These measures are especially advantageous because the planning of the equipment to be accommodated in the deck for the work to be effected with the platform requires a long period of time during which the manufacture of the foundation body and of the support legs and the assembly thereof can already take place. 4497» - 9 The spurs which are arranged to be pushed in can, after their penetration into the sea bed, be used as guiding means for lowering and lifting the foundation body in shallow water.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein:Figure 1 is a side elevation (partly in section) of a deep water platform for oil-wells, having a foundation body shown in the lowered condition? Figure 2 is a plan view of the foundation body shown in Fig.1, taken along the line Figure 3 shows a vertical partial section of Figure 2 taken along the line III-III, and drawn to a larger scale? Figures 4,5,5 and 7 show detailed views of areas IV, V, VI and VII of Figure 3 drawn to a larger scale; and Figures 8, S, 10 and 11 are diagrammatic elevation views showing the assembly of a platform according to the invention, in four different phases of construction. 2o Referring first to Figures 1, the oil rig comprises a platform 10 and a foundation body 17, the platform 10 being adapted to be used as drilling island for the drilling of oil in low-lying sea beds. The platform comprises a buoyant deck II which carries the working devices, such as drilling derricks 13, cranes 14 and accommodation rooms 15 for the workers, and which is suspended for vertical displacement from four tubular supports 16 anchored in a foundation body 17 which is also buoyant. Referring to Figure 2, the foundation body 17 comprises a plurality of cells 18 which are laterally connected with each other and which enclose a large caisson 19 which is arranged to receive ballast in the form of sand, mud or the like ballast materials and which can be partly flooded and pumped out again. The cells 18 of the foundation body 17 can be filled with water or oil and flooded or pumped out and constitutes together with the caisson 19 a single closed foundation body 17 having a large basic area which is set down on the sea bed 20.

In order to avoid erosion under the base surface of the foundation body 17, there are attached skirts projecting beyond the base surface 21 of the foundation body 17 and which, are made fr.'.m a relatively thin sheet steel In order that they can penetrate into the sea bed 20.

In order to stabilize the foundation body 17 against lateral displacement in the last phase of the lowering process, a spur 24 is arranged in gussets 23 between the four supports 16 and the cells 18 which enclose them externally. The spurs 24, of which also a plurality may be provided distributed over the entire base surface 21 of the foundation body 17, project on the foundation body 17 being lowered perpendicularly beyond the said base surface 21 and are arranged to penetrate into the sea bed 20 before the skirts 22 reach the sea bed 20.

Referring now to Figure 3, each spur 24 comprises a steel tube 26 which is open at its lower end 25, and is sealed at its upper end 27 with a hemispherical cap 28.

Ths upper portion of the tube 26 is divided-off by a partition 28^ and constitutes an air-filled buoyancy chamber 29 through which extends a tubular pressure compensating line 30 which is welded into the hemispherical cap 28 and into the partition 28^.

Each spur 24 is arranged in a cylindrical guiding shaft 31 adapted to its cross-section. The guiding shaft 31 is open at its lower end 32 and at its upper end 33 is sealed with a hemispherical cover 34 made from steel, which Is secured to be pressure-tight and releasable, with an annular flange 35, at an upper projection 36 of the guiding shaft 31. The cover 34 bears on a support construction 37 mounted on the foundation body 17 and which is constituted by steel girders 38 and 39 arranged cross-wise, and circularly cut-out support plates 40 bearing at the outer periphery of the hemispherical cover 34.

The pressure compensating line 30 extends through the cover 34 of the guiding shaft 31 and is sealed relative · 11 to the cover 34 by means of a stuffing box 41. The stuffing box 41 is shown to a larger scale in Figure 7 and comprises a steel ring 42 which is C-shaped in cross-section τ and which is secured to a compensating plate 35 welded to the cover 34 and surrounds the pressure compensating tube 30 in spaced relationship. A gap 43 between the ring 42 and the outer face of the pressure compensating tube 30 is filled with an elastically or plastically deformable mass 44 which is compressed from above by the lower edge 45 of an angular pressure ring 46 which abuts against the pressure compensating tube 30 and is arranged to be pulled with screws 47 and nuts 48 in the axial direction of the pressure compensating tube 30 against the upper flange 49 of the C-shaped ring 42.

Secured to the upper end 50 of the pressure compensating tube 30 is a lifting strap 51 to which can be connected a lifting rope 52 of lifting gear {not shown) using a shackle 53. The guiding shaft 31 limited at the upper side by the cover 34 is so long that it is able to receive the spur 24 for such a distance that the lower end thereof does not project beyond the base surface 21 of the foundation body 17. The walls of the guiding shaft 31 are like the remaining elements of the foundation body 17 , made from reinforced concrete and can be lined with a steel tube or surrounded by a steel tube. Referring to Figure 4, at the inner face 55 of the wall 54 of the guiding shaft 31 there are arranged in spaced relationship from each other a plurality of guiding and supporting rings 56 made from steel and which project slightly beyond the inner face 55 of the wall 54 and guide the cylindrical tubs 26 at the external periphery thereof, retaining it in spaced relationship relative to the inner face 55 of the wall 54. Referring to Figure 6, the uppermost guiding and support ring 56 co-operates with a carrier ring 57 which is laid about the spur 24 at the upper end 27 and with which the spur tube 26 bears in the lowered condition on the uppermost guiding and supporting ring 56.

Arranged between the two upper guiding and support rings 56, at the inner face 55 of the wall 54 of the guiding shaft 31, is a packing device 58 which provides a seal at the outer periphery of the spur tube 26 and seals off the upper element of the guiding shaft 31 relative to the lower element thereof, so that a pressure chamber 59 is formed between the cover 34 of the guiding shaft 31 and the spherical cap 28 of the spur tube 26.

The packing device 58 is shown in greater detail 10 in Figure 5. It comprises a plurality of chambers 60 arranged one above the other in the axial direction of the guiding shaft 31 and which are sealed by end walls 61 relative to the guiding shaft 31 and separated from each other by partitions 62. Arranged in each chamber 60 is a packing ring 63 made from resilient material, for example from rubber, and which is pressed by a pressure hose 64 against the outer face of the spur tube 26. The pressure hose 64 is also located in the chamber 60 and abuts against the outer periphery of the packing ring 63. The interior of the pressure hose 64 is connected to a pressure line 66 containing a valve 65 through which a pressure medium, for example pressured water or pressure air, can be passed into the interior of the pressure hose 64 which causes its expansion and thereby presses the sealing ring 63 against the wall of the spur tube 26. The magnitude of the air pressure in the pressure hoses is reduced so that the pressure chamber 59 filled with pressure water is sealed but simultaneously sliding of the spur 24 along at the packing device 58 is possible. On pressing the sealing rings 63 against the wall of the spur tube 26, the entire packing device 58 Is vented or discharged with water through bores (not shown) arranged in the chamber walls 61 and 62. The bores are connected with lines 75 indicated by broken lines and extending on the external air supply (not shown].

Referring again to Figure 7, secured to the cover of the guiding shaft 31 are:- an air line 67 having a ventilating and venting valve 68; and a water line 69 which forks to afford an inflow line 71 leading to a pressure water pump (not shown) and having a filling valve 72, and a discharge line 70 leading to a receiving vessel (not shown) and into which an adjustable pressure limiting valve 73 is fitted. The pressure limiting valve 73 is also designed as a shut-off valve.

Subsequent to completion of the foundation body 17 in the dock, the pressure hoses 64 and the sealing rings 63 are installed in the packing device 58 and the pressure hoses 64 are connected to the air supply. Thereupon the spur tube 26 is introduced from above into the guiding shaft 31 until the carrier ring 57 at the spur tube 26 bears on the uppermost guiding and support ring 56. Thereupon, the shaft cover 34 is pushed over the pressure compensation tube 30 and is secured with its flange 35 at the upper end 33 of the guiding shaft 31. Then, the stuffing box is tightened in the lifting chamber cover and the pressure compensation tube 30 is secured to the rope 52 of the lifting and lowering winch. Employing the lifting tackle, with the venting valve 68 open and while simultaneously venting the .pressure chamber 59, the spur tube 26 is pulled up to such a level that the lower end 25 no longer projects beyond the base face 21 of the foundation body 17. The filling line 71 is than subsequently connected to the pressure water pump and the discharge line 70 to the receiving vessel. Therewith, the spur 24 is finally mounted and the platform can be launched into the water and floated to the site of use. Tn this connection, it is important that the spur 24 does not project beyond the base face 21 of the foundation body 17 and doas not enlarge the immersion depth thereof which in any case is already very considerable.

On reaching the desired site and prior to lowering of the foundation body, the spur tube is, with simultaneous venting of the pressure chamber 59, lowered to such an extent by means of the lifting tackle, into the guiding 44072 - 14 shaft 31 that the carrier ring 57 of the spur tube 26 once again bears on the uppermost guiding and supporting ring 56. Thereupon, the lifting tackle is relieved of load.

The pressure hoses 64 are subjected to the action of air pressure which presses the sealing rings 63 against the outer wall of the spur tube 26. Thereupon the pressure chamber 59 is, with simultaneous venting by the air line 67, filled with water. Thereupon, the shut-off valve 68 is closed and the pressure-limiting valve set at that pressure which on being multiplied by the cross-sectional area of the spur tube 26 affords the maximum vertical load which the spur 24 is able to and should take up.

After the foundation body 17 has been lowered on to the sea bed 20, the spurs 24 whioh now project beyond the base surface 21 and beyond the under edges of the skirts 22 initially impinge on the sea bed and penetrate, under the load of the buoyancy of the foundation body 17 into the sea bed, thereby stabilizing the foundation body 17. The spurs 24 thus take-up the horizontal forces exerted on the foundation body 17 due to the strong horizontal streams between foundation body 17 and sea bed 20 and transmit them into the sea bed. The foundation body is in this way simultaneously nailed fast So to speak at the sea bed and can no longer perform any horizontal movements when the skirts 22 reach the sea bed and begin to press into the latter.

If the peak resistance and the generated surface friction at the spurs 24 become so considerable that the latter are no longer able to readily press into the sea bed under the load of the underdrive the pressure increases in the pressure chamber 59 of the guiding shaft 31. The pressure limiting valve responds to this pressure increase and allows as much water to flow out of the pressure chamber 59 into the receiving vessel connected to the line 70 so that the pressure in the pressure chamber 59 is maintained constant. This guarantees that the vertical load associated - 15 with the spur 24 of the entire underdrive of the foundation body 17 is not exceeded and damage of che foundation body 17 at the securing locations of the spurs 24 cannot occur. In the last phase of lowering, the guiding chambers push, themselves over the spurs 24 which thereby slide into the guiding chamber;:· until the steel skirts 22 penetrate into the sea bed 20.

At this instant in time, the shut-off valve 72 is opened, so that water is able to flow away freely out of the pressure chamber 59 and the pressure in the chamber 59 falls to zero. The entire underflow of the foundation body 17 now acts only on the steel skirts 22 so that the latter, no longer decelerated by the spurs 24, are able to penetrate into the sea bed 20 until the entire base surface 21 of the foundation body has been set down on the sea bad 20.

It will be appreciated that it is possible to once again lift the foundation body 17, with fche aid of the spurs 24, from the sea bed, the pressure limiting valve being closed and water being pumced through the valve 72 under pressure into the pressure chamber 59. This pumpingin of water takes place- simultaneously with pumping out of the buoyancy chambers of the foundation body, since the spurs 24 are of course not intended to push up the entire load of the foundation body but only to promote liftingoff from the sea bed in which the foundation body 17 has, after a relatively long time, so to speak sucked itself fast and from which it is difficult to release it even employing powerful buoyancy forces.

When the foundation body 17 has then released itself from the sea bed, on further sliding upwardly through the water if pulls the spur tubes 26 out of the sea bed 20 and these then bear with the carrier rings 57 on the upper guiding and supporting rings 56. As soon as the spur tubes are free, they can be pulled back with the aid of the lifting tackle at the ropes 52 once again into their guiding shafts 31, for which purpose the valve 73 is opened, so that the water is able to flow out of the pressure chamber 59 into the receiving vessel.

It will be appreciated that, on penetration of the spur tube 26 into the sea bed 20, no water pressure is able to build up in the interior of the spur tube, since the water enclosed in the lower portion of the spur tube is able to escape upwardly through the pressure compen sation tube 30. The buoyancy chamber 29 ensures that the weight of the spur tube 2(j is compensated and no high forces are necessary for lowering the spur tube under water in the guiding tube 3l and for lifting it.

Figures 8 to 11 show a preferred method for the assembly of the platform. After manufacture, the foundation body 17 is launched from the dock and transported to a location in the vicinity of the building site or shipyard which is manufacturing the support and the deck with the equipment to be accommodated therein. There the foundation body is lowered on to the sea bed in shallow water, the spurs 24 penetrating into the sea bed 20. As soon as the point and generated surface friction of the spurs in the sea bed has become adequate to take up completely the load of the foundation body 17, the foundation body is lowered to such an extent that the surface 80 of the foundation body 17 is below the water surface 81 by the amount t (Figure 9), the amount t being at least as large as the immersion depth T of the deck 11 (Figure 10).

On lowering, the foundation body is guided at the spurs 24 which, prior to immersion of the surface 80 of the foundation body into the water are pressed into the sea bed 20 and on the point and generated surface friction becoming larger are pushed into the guiding chambers.

Assembly of the supporting legs 16 is expediently already begun as long as the surface 80 of the foundation body 17 still projects over the water surface.

On lowering the foundation body , the spurs 24 assume the task of guiding and stabilizing the foundation 44973 member 17, which., on immersion in the water being completed, passes into an unstable floating position.

Then subsequently the deck II, which, is manufactured at the shipyard in a plurality/ of elements, expediently a middle element and two lateral elements, is floated between the support legs 15 over the foundation body 17 and assembled on the latter, after the foundation body 17 has once again been lifted to the water surface 81 by the spurs 24. In this manner, it is possible to assemble the deck 11 only subsequent to completion of the support legs 16, whereby time is obtained for manufacturing the deck and for equipping tha support legs 16.

The invention, is not limited to the example of embodiment. For example, it is also possible to provide solid spurs or to manufacture the guiding shaft from a steel tube. Furthermore, the spurs need not be circular and may also have some other cross-sectional shape, if this should be found to be expedient. It may also be expedient to design the packing or sealing devices differ20 ently from that shown in ths drawings.

Claims (26)

1. CLAIMS:1. A deep water platform for marine work, having a foundation body which is positioned on the sea bed and at least one vertical spur directed towards the sea bed and projecting beyond the base face of the foundation body, wherein the spuk is arranged, to be sealingly and axially displaceable in a guiding shaft adapted to its cross-section, which said guiding shaft is sealable at its upper end and forms with the spur a pressure chamber having an adjustable pressure limiting valve.

2. A platform according to claim 1, wherein the pressure limiting valve is fitted in a line which connects the pressure chamber with a pressure liquid container.

3. A platform according to claim 1 or 2, wherein the pressure limiting valve is designed also as a shut-off valve and wherein a filling and discharging valve is connected in parallel with it.

4. A platform according to any one of the preceding claims, wherein a ventilating and venting valve is arranged at the upper end of the guiding shaft.

5. A platform according to any one of the preceding claims, wherein the spur and its guiding shaft have cylindrical cross-section and are hemispherical at their upper ends.

6. A platform according to any one of the preceding claims, wherein the guiding shaft is sealed by a hemispherical cover which is secured to be pressure-tight and releas able at the guiding shaft and bears at a supporting structure mounted at the foundation body.

7. A platform according to any one of the preceding claims, wherein air medium and pressure medium lines are connected with associated valves to a hemispherical cover of the guiding shaft.

8. A platform according to any one of the preceding claims, wherein the wall and/or the hemispherical cover of the guiding shaft are made from concrete.

9. A platform according to any one of the preceding claims 1 or 7, wherein the wall of the guiding shaft and its cover are made from steel.

10. A platform according to any one of the preceding claims 1 to 7, wherein the wall cf the guiding shaft is lined with a steel tube and/or surrounded by a steel tube.

11. A platform according to any one of the preceding claims, wherein there is provided at the inner face of the wall of the guiding shaft at least one annular chamber in which there is located a packing ring of resilient material which is acted upon by a pressure hose arranged at its outer side and itself acted upon by a pressure medium so that the packing ring is urged against the outer periphery of the spur.

12. A platform according to any one of the preceding claims, wherein a plurality of chambers having clamping and packing rings is arranged one behind the other in the axial direction of the guiding shaft.

13. A platform according to any one of the preceding claims, wherein there are arranged at the inner face of the wall of the guiding shaft guiding and supporting rings which guide the spur and retain it in spaced relationship relative to the wall of the guiding shaft.

14. A platform according to claim 13, wherein the •1 97¾ spur has at its upper end a carrier ring laid about its outer periphery and which co-operates with one of the guid· ing and supporting rings.

15. A platform according to any one of the preceding claims, wherein the guiding shaft is so long that it is able to receive the spur at least to such an extent that the lower end thereof does not project beyond the base face of the foundation body.

16. A platform according to any one of the preceding claims, wherein the spur is suspended from a traction means which extends sealingly through the cover of the guiding shaft and at which lifting gear engages.

17. A platform according to any one of the preceding claims, wherein the spur is a tube which is open at its lower end and the interior of which is connected by a pressure compensating tube with the external water or a receiving container.

18. A platform according to any one of the preceding claims, wherein the spur tube has an air-filled buoyancy chamber.

19. A platform according to claim 17, wherein the pressure compensating tube is constituted by a tube which extends through the pressure chamber of the guiding shaft and the buoyancy chamber of the spur and is slidingly sealed at the cover of the guiding shaft and simultaneously serves as traction means for the spur at which lifting gear engages.

20. A platforh according to claim 17, wherein the pressure compensation tube is sealed at the cover of the guiding shaft with a stuffing box.

21. 21. A method for assembly of a deep water platform for marine work of the type having a foundation body adapted to be positioned on the sea bed and having at its base face spurs adapted to be pushed in vertically, support legs secured to the-said foundation body, and a buoyant deck arranged to be vertically displaceable at the support legs, including the steps of: positioning the foundation body with its load-dependently displaceable retaining spurs in shallow water on the sea bed effecting penetration of the spurs into the sea bed; lowering the foundation body to such an extent, guided at the spurs, that the upper surface thereof is submerged more than the amount of the immersion depth of the buoyant deck below the water surface; floating the deck over the foundation body; raising the foundation body to the water surface; and securing the support legs to the foundation body.

22. A method according to claim 21, wherein the deck is floated in a plurality of buoyant elements over the foundation body and is assembled thereupon.

23. A method according to claim 21 or 22, wherein the support legs are, prior to floating in of the deck secured to the foundation body and the elements of the deck are floated between the upwardly projecting support legs over the foundation body.

24. A method according to any one of the preceding claims 21 to 23, wherein the spurs after penetration into the sea bed, are employed as guide means and for lowering and lifting the foundation body.

25. a deep water platform for marine work, constructed substantially as herein described with reference to and as illustrated in the accompanying drawings.

26. A method of assembly of a deep water platform for marine work, substantially as herein described with reference to Figs. 8 to 11 of the accompanying drawings.