WO1985004682A1

WO1985004682A1 - Gravity platform for great water depth, method of manufacturing same, and use of such a platform

Info

Publication number: WO1985004682A1
Application number: PCT/NO1985/000020
Authority: WO
Inventors: Per Bull HAUGSÖEN
Original assignee: Haugsoeen Per Bull
Priority date: 1984-04-06
Filing date: 1985-04-02
Publication date: 1985-10-24
Also published as: AU4210285A; BR8506602A; NO841368L; EP0178311A1

Abstract

A gravity platform for a great water depth, wherein the platform in operational condition is intended to stand stably on the sea bed (3) essentially by its own weight and in this position is intended to project above the water surface (4) for support of the deck structure (5). The platform comprises a lower part in the form of a base (6, 7) and a caisson structure (1) of concrete, and an upper part in the form of a framework structure (2). In order to take into account natural circumstances and conditions in an optimal manner, i.e. existing limitations in available towing draft out from sheltered coastal area to the site of installation, the framework structure (2) as well as the caisson structure has a height of at least 100 meters, and the monolithic caisson structure (1) has such a great height in relation to the framework structure (2) that the centre of gravity of the platform is located substantially below its centre of buoyancy in a floating position during towing-out at a depth which is essentially equal to or greater than the height of the caisson structure (1). Thus, the platform is floating-stable at the depth in question without any external means.

Description

Gravity platform for great water depth, method of manufacturing same, and use of such a platform

The invention relates to a gravity platform for a great water depth, wherein the platform in operational condition is intended to stand stably on the sea bed essentially by its own weight and in this position is intended to project above the water surface for support of a deck structure, the platform comprising a lower part in the form of a base and a caisson structure of concrete, and an upper part in the form of a framework structure carried by the caisson structure. Further, the invention relates to a method of manufacturing such a platform, and use of a gravity platform of the stated type.

There are previously known gravity platforms which are of the stated type, but which are intended for use at shallow water. With the concept "shallow water" in this connection is meant two circumstances. One of these is that the water depth at the site where the platform is to be installed, is so small that the platform is capable of being towed out from sheltered coastal area in the most favourable towing route with a draft which is not limited by the smallest water depth in the towing route. The other circumstance concerns the wave action which, at shallow water, to a far greater extent manifests itself to a depth close to the sea bed.

Gravity platforms for shallow water very often are characterized by a concrete caisson consisting of a plurality of cells, and hollow columns projecting upwards from the caisson. Usually, the columns will partly be submerged during towing-out of the platform so that, during the towing-out phase, they contribute to stabilizing the platform. Both the concrete caisson and the columns will be able to be subjected to substantial wave forces. This principle is that which is mostly utilized on gravity platforms in the North Sea today. For the columns, also the circumstance applies that even if they are wave-loaded as a result of the volumes they possess, the same volumes to a substantial degree provide a positive contribution to supporting useful load during towing-out to the site of installation.

A typical gravitation platform for shallow water is "Condeep G3™ from Norwegian Contractors. This platform has substantial parts of the buoyancy volume at a high level. This shows that the requirement for reducing the wave forces in the upper part of the structure usually is not particularly great at shallow water, and that - from a total evaluation - it may be directly desirable to have a great volume at a high level, volume and water plane area giving a positive contribution to useful load on the deck during the towing-out.

When constructing gravity platforms for a great water depth the determining conditions are substantially different from the conditions at shallow water. The concept "great water depth " as used in this connection, covers two circumstances► One of these is that the water depth at the site where the platform is to be installed, is so great that the platform has to be towed out from sheltered coastal area without the greatest possible towing draft of the platform being able to be used, as a result of the depth limitations i_n the towing route which is used. This is the situation for the topical platforms which at present are being planned for deep areas of the North Sea . (south of 62 North) , for example in the "Troll" field. The other of^"the said circumstances is the relation of the platform to the influence of the most dominating waves. In shallow waters the wave energy will, as mentioned, to a substantial extent propagate down to depths close to the sea bed. Roughly estimated it may be said that the wave energy is without any substantial importance at depths greater than half the wavelength, wherein the wavelength L may be expressed as

2 L = 1,56 x T , wherein T is the wave period in seconds. For wave periods of ca. 16 seconds, which may be said to represent the greatest loads ,one will then see that those parts of the platform structure being below a water depth of ca. 200 m, will be not very influenced by wave forces.

Gravity platforms for a great water depth have so far not been installed. However, several types are known from the literature. As an example, there may be mentioned a platform with the designation "T300" from Norwegian Contractors. This is a pure concrete platform for deep water and consists of a caisson base, oblique columns, a so called "riegel" and one or more columns carrying the platform deck. A relatively large part of the structure projects above the water line during towing-out, and the structure is subjected to large wave forces in the operational condition.

There is also known a steel platform consisting of three or four "flasks" connected to stiffening struts

(Tecnomare) . ^" The separate "flasks" provide for buoyancy and stability, and the steel framework has for its task to keep the flasks together. Here is not used any lower monolithic caisson structure,- as in the present platform. The principle of a caisson structure and a separate frame¬ work on the top thereof does not apply for the Tecnomare platform, either.

Further, in the periodical "Offshore", July 1975 (page 40) , there is described a platform having a low concrete base, a plurality of extended concrete cylinders being connected to the base, and a steel framework structure which is built directly on the low concrete base. For great ocean depths it is here required a corresponding framework structure which would have been substantially higher than in the present platform wherein the framwork with small adjustments is to be placed on top of the caisson structure as an extension thereof.

It is a primary object of the present invention to provide a gravity platform for a great water depth which, totally seen, is more attractive than alternative platforms, both with respect to building time and price, and which in an optimal manner takes into account inherent circumstances and conditions, and which also gives the best possible result when taking into consideration the existing, physical limitation in available towing draft out from sheltered coastal area.

For the achievement of the above-mentioned object there is provided a gravity platform of the introductorily stated type which, according to the invention, is characterized in that the framework structure as well as the caisson structure has a height of at least 90 meters, and that the caisson structure is amonolithic structure of such a great height that the centre of gravity of the platform is located below its centre of buoyancy in a floating position during towing-out at a draft which is essentially equal to or less than the total height of the caisson structure, so that the platform is floating-stable without external means.

According to the invention there is also provided a method of manufacturing such a platform, which method is characterized in that the framework structure and the caisson structure are built separately at respective sites, and that at least a part of the framework structure as an essentially finished unit in a floating position is brought into position to be connected to the caisson structure which is in a suitably submerged floating position for said interconnection.

An important aspect of the present invention also consists in the use of a gravity platform which in operational condition is intended to stand stably on the sea bed essentially by its own weight and in this position is intended to project above the water surface for support of a deck structure, the platform comprising a lower part in the form of a base and a caisson structure of concrete, and an upper part in the form of a framework structure carried by the caisson structure, at a water depth which is so great that the wave action in the depth region in which the caisson extends is unessential in relation to the wave action in the depth region in which the framework structure extends, the caisson still being so high that it contributes to the quite substantial portion of the floating stability of the platform during towing thereof without any external stabilizing means.

The present invention implies several substantial advantages which are to be further discussed below. It turns out that, in deep water, platforms consisting of the combination of a caisson and a framework ("hybrid" platforms) are far more favourable than in shallow water. This can be shown by general considerations, and it can also be shown by calculation examples. One of the circumstances which comes in, is that the framework structure will be subjected to smaller wave forces than one or more towers which are only connected to each other at deck level. The caisson structure thereby becomes subjected to smaller horizontal forces and moments than what is the case with other known solutions. This makes possible a smaller and cheaper caisson and associated base. It is particularly important to reduce the forces on the uppermost part in the case of the deep water platforms because of the fact that the moment arm on the bases of such platforms is especially large. In shallow water platforms this is not equally important, and in addition shallow water platforms do not have a high caisson providing a stabilizing effect in an equally effective manner as in deep water platforms. In shallow water platforms it it therefore also to a greater degree desirable with volume in the towers.

As mentioned it can be shown by calculation examples that platforms with the combination of a caisson and a framework is far more favourable in use on deep water than in use on shallow water. In the following there is shown a calculation of a so-called efficiency factor illustrating the difference between deep and shallow water.

In the calculation it is presupposed that the platforms for shallow and deep water are to carry the same useful load on the deck during towing to the site of installation. Further, it is presupposed that it is decisive to minimize the horizontal forces on the platform. Therefore, the following calculation example is performed with the horizontal force as a criterion of whether a favourable platform is under consideration.

The first calculation example relates to a shallow water platform with a tower where the water depths is 150 m and the caisson height is chosen equal to 60 m. The dimensioning wave height is 31 m with a period of 17 seconds.

Horizontal wave acceleration at the top of the caisson is

2 ca. 0.70 m/s , and the wave acceleration in the middle of

2 the tower (105 m above the sea bed) is ca. 1.20 m/s . Top ballast in the caisson is supposed to be located ca. 20

^• above the bottom. It is here to be shown how the effect of a raising of the caisson with 1 m is as compared to an

3 increase of the tower volume with 1 m . The mass coefficient of the tower is set equal to 2.0 and for the caisson equal to 1.5. The efficiency factor E is by definition:

_E_ Stabilizing moment during towing

Horizontal wave force for installed platform

3

For raising of the top of the caisson with 1 m :

_TΓ. _ (60 m - 20 m) ^» 1 ton _ ,_Q ^Ξs 1.5 • 0.70 ³⁸

For an expansion of the tower in the central area with 1 m

v _E -- ((110055mm-- 2200 mm)) ^»- 1 ton _ ₃₅ ^fit 2.0 • 1.20

fa - ³⁸ - i i

Ξ_t ^" 35 ¹'¹

In this example the ratio between the efficiency factors is ca. 1.0. This means that, for a platform wherein the horizontal force is of substantial importance for geotechnical stability, it is of little importance whether the caisson is made higher or whether the tower is expanded in the central area.

The second calculation example applies to a deep- water platform with a tower where the water depth is 300 m and the caisson height is chosen equal to 200 m, so that the central point of the tower is ca. 250 m above the sea bed. Dimensioning wave height is 31 m with a period of 17 seconds. The wave acceleration at the top of the caisson

2 is ca. 0.14 m/s , and in the central area of the tower

2 ca. 1.05 m/s . The efficiency factor is calculated as in the example of the shallow water platform.

3 For raising of the top of the caisson with 1 m :

■_■. (200 - 50) » 1 ton _ _, . ^Es ⁼ 1.5 • 0.14 ⁷¹⁴

For an expansion of the tower in the central area with

1 m^"

E_ = _ (250 - 50) * 1 ton = 95

2.0 • 1.05

^Es _ 714 _ _ E_t ^" 95 ^{~ /}'⁵

The illustrated examples show that when a platform is to have a certain stabilizing capacity during towing to the site of installation, and at the same time is to have minimalized horizontal forces subsequent to installation, this is optimalized in different manners for a shallow water platform and a deep water platform. For a shallow water platform, the effect of an adjustment of the caisson or an adjustment of the tower volume is rather equal. Whether the above-mentioned ratio is 0.5 or 2.0 is of little interest. For a deep water platform, however, this ratio has changed quite substantially. And this applies even if no limitation exists in the towing draft of the platform (towing draft limitation) .

There exists a still further circumstance which makes the combination of caisson and framework favourable, viz. the phase shift of the greatest wave forces.- The caisson is a volume structure which to a dominating degree is influenced by forces of inertia from the water particles. For a framework structure with relatively small volumes, forces resulting from water particle velocity will be dominating. The maximum values of these loads are quite out of phase in relation to each other. Totally seen, this gives a favourable load situation for the platform, even if the greatest force comes in an intermediate phase with some contribution from each type of load. In principle,- it might also be favourable to utilize this effect in shallow water platforms, but here it will be seen from the shown calculation example that the volumes in columns are of a positive value to a far greater extent than in deep water platforms. It is therefore in deep water platforms that said phase shift effect can be utilized with a considerable efficiency, without loosing other favourable effects. The foregoing calculations show how it is favourable to establish buoyancy in connection with the caisson instead of in the towers in deep water platforms. In the platform according to the invention one has, to the practically greatest possible degree, taken the consequences thereof. However, the present platform structure becomes even more favourable when a towing draft limitation exists. For a platform which is to be built in Norway for use on deep water, it applies that there is a towing draft limitation of 220-250 m during towing-out from sheltered, coastal area. This means that possible volumes in towers cannot be utilized for buoyancy and stabilization, and a good platform to an even greater extent must be constructed with the smallets possible volume in the uppermost part. In the present plat¬ form this is complied with by the use of a framework structure in the stated manner. The improvement obtained applies independently of the towing draft limitation, as - shown by the efficiency factor calculation, but becomes even more favourable when there is a towing draft limitation. Here also the weight of towers as compared to that of a framework structure comes in as an important consideration. A framework structure will be lighter. This means that, in the case of the towing draft limitation, the caisson is to establish smaller buoyancy and stability when a framework is used. This means a smaller caisson, with the positive effects resulting therefrom.

The foregoing considerations show that the present gravity platform for a great water depth in an optimal manner takes into account inherent circumstances and conditions, and that, by going new ways with respect to structural solution for deep water platforms, one has obtained a special adaptation to the depth conditions in the North Sea and the practical towing draft limitations^' from sheltered coastal area at the Norwegian coast.

In previously known platform structures intended for use on deep water, such as the aforementioned "T300", only one column is used in the latest versions. This is partly to reduce the total negative effect due to weight and wave stresses, as compared to alternatives with several columns. On account of the deck structure it is, however, desirable to have a larger base or foundation than one column is able to give. The framework structure meets with this requirement without corresponding negative effects.

An important circumstance which comes in addition to other attractive circumstances is that a framework structure may be built simultaneously with the caisson structure and be placed either finished or in several parts on the top thereof. This is a substantial contribution to a reduction of the building time which in known concepts is more than 4 years. The long building time is a great disadvantage. At great water depths, the inspection problems for pure steel platforms may also be great. By using a frame¬ work in the zone which is relatively conventional with respect to inspection, and concrete - which must be said to be free of maintenance - farther down, one will thereby have a platform which is favourable viewed from a maintenance point of view, as compared to pure steel platforms.

An advantageous embodiment of the platform according to the invention is characterized in that, in connection with the framework, there is provided at least one hollow, air-filled column extending at least between the top of the framework structure and the top of the caisson structure and providing at least temporary dry access to the lower part of the platform, and which is arranged for exercise of operational control during different phases, such as assembly, towing, installation and operation of the platform.

The hollow column may be arranged in several different _ D manners. It may be a seσarate column which is outside of the framework structure and which is braced thereagainst, or it may be disposed and braced inside of the panels of the framework structure. The hollow column may also be a part of the framework structure and it may be a hollow leg,

«« either of concrete or steel.

The platform structure according to the invention may • also comprise combination solutions wherein the above- mentioned advantages are achieved in combination with a tower structure projecting upwards from the caisson structure. 5 In addition to the tower structure there will be columns having the support of the cantilevered tower structure. Thereby the tower structure may be reduced to the smallest possible size as the necessary support for carrying deck structures and the like to a substantial degree will take place by 0 means of the columns. Risers- and the like may be placed outside of the tower structure. When the platform includes such a tower which is cantilevered from the caisson structure and supports surrounding columns, the tower will also form an access shaft to the lower part of

25 the platform.

In the platform there may be provided a dry compartment in the lowermost part of the caisson, where the compartment constitutes an extension of the dry access shaft. The compartment is used to pull in tubes from the outside of

30 the platform by means of known devices.

I connection with the platform there may be arranged casings which outermost consist of conductors curving out from the vertical already before reaching down to the sea bed. This may be done in several manners. One

35 manner is to let the tubes go down through the caisson and let them have an outward curve whichis as great as possible. Another manner of arranging the casings is to lead them downwards through the framework structure or on the out- side thereof and let them go further down on the outside of the caisson structure against which they are braced The tubes may extend through a corbeled structure, but they may also extend completely on the outside. The method has advantages as compared to known methods. The circum¬ stance that the tubes do not pass through air-filled buoyancy chambers means that no boring-out of concrete plugs is to be carried out, such as for the installed Condeep platforms. Further, this is advantageous as compared to known solutions, in that the tubes may be curved out to a substantial degree above the sea bed. The combination of framework and caisson structure means that the curving-out of the tubes may be substantial already at the top of the caisson structure. This is also an improvement as compared to known platform alternatives for deep water wherein the tower geometry poses limitations. For some petroleum reservoirs it is of great importance to establish the tube-curving-out as high up^' as possible. Conductor, pipes carried obliquely outwards from a structure and. into the ground may be subjected to adverse compulsory deformations in settling of the platform. As a special measure here it may be desirable to have an outer guide tube for the conductor pipes allowing the conductor pipes a certain flexibility at a level directly above and possibly just below sea bed level. This will be able to reduce stresses due to settling deformations.

.The caisson structure normally will be built up by a number of upstanding cells, and the conductor pipes may then pass through the top domes and the bottom domes of the cells. They may here extend in guide tubes creating a seal between external water and the interior of the cells. They may also be guided through the cells after installation in the field in that concrete plugs are bored out in the domes. A_{S a} special measure when the platform is installed in connection with pre-drilled wells, the lowermost part of the conductor pipes may be curved. This will cause a continued curving after the conductor pipes have been carried down into the loose masses, and the total stresses in critical zones will be reduced.

The invention will be further described below in connection with a number of the exemplary embodiments with reference to the drawings, wherein like reference numerals designate like or corresponding parts in the different Figures, and wherein

Fig. 1 shows a side view of a first embodiment of a platform according to the invention; Fig. 2 shows an enlarged section along the line II-II in Fig. 1;

Fig. 3 shows a section along the line III-III in Fig.l;

Fig. 4 shows a side view of a second embodiment of a platform according to the invention; Fig. 5 shows a section along the line V-V in Fig. 4;

Fig. 6 shows a side view of a third embodiment of a platform according to the invention;

Fig. 7 shows a side view of a fourth embodiment of a platform according to the invention wherein the caisson h s only one cell;

Fig. 8 shows a cross-section of the caisson along the line VIII-VI I in Fig. 7;

Fig. 9 shows a side view of a fifth embodiment of a platform according to the- invention; Fig. 10 shows a section along the line 10-10 in Fig.9;

Fig. 11 shows a longitudinal partial section of an embodiment wherein a portion of the base of the platform is raised for the formation of a space for pre-drilled wells;

Fig. 12 shows a section through a cell configuration; Fig. 13 shows an embodiment of an interconnection between a caisson structure and a framework structure;

Figs. 14-16 show three different phases of a first embodiment of the method according to the invention for manufacturing the platform according to the invention; Fig. 17 shows a second embodiment of the method according to the invention;

Figs. 18-21 show different phases of a third embodiment of the method according to the invention; Figs. 22-24 show different phases of a fourth embodiment of the method according to the invention;

Figs. 25-27 show different phases of a fifth embodiment of the method according to the invention;

Figs. 28-30 show different phases of a sixth o embodiment of the method according to the invention; and

Figs. 31-32 show different phases of a seventh embodiment of the method according to the invention.

The gravity platform shown in Fig.l consists of a lov/er 0 part in the form of a caisson structure 1, and an upper part in the form of a framework structure 2. The platform is placed on a sea bed 3, and the framework structure projects above the water surface 4 and supports a deck structure 5. 5 In the illustrated embodiment the caisson structure 1 is composed of a plurality of cells cast together to a monolithic structure. The cells constitute a number of cell groups of which an outer, low cell group 6 and an inwardly located, somewhat higher cell group 7 form a o base structure resting on the sea bed 3. A skirt 8 constituted by circular cell walls in concrete or steel, depends from the base and penetrates the sea bed. The base or foundation may have a relatively large area, for

2 example 5000-20000 m , and parts of the base may have low 5 height, e.g. less than 30 m.

Parts of the base are extended upwards in the form of a central cell group constituting the upper part of the caisson and consisting of vertical cells 9. As shown in Fig. 2, the cells 9 are of round cross-section and are 0 cylindrically shaped, but they may also have another cross- sectional shape, for example polygonal, as shown in Fig. 12. The height of the cell group 9 is substantially greater than the width. The number of cells and their mutual arrangement may vary. The cell diameter may vary when there are several 5 cells.

The cells of the cell groups 6 and 7 of the base structure are closed at the top by curved shell structures 10, and also the high, central cells 9 are closed by corresponding shell or plate structures (domes) 11.

In the illustrated embodiment the framework structure 2 of the platform consists of four columns 12 carried by the corner cells of the cell group 9, and a number of schematically shown, horizontal struts 13 and oblique struts 14.

The horizontal struts 13 extend between the columns 12 as shown in Fig. 3. The columns 12 carry the deck structure 5, and at least one of the columns may be a hollow, air-filled column providing dry access to at least the uppermost part of the caisson 1. Thus, the column contains a shaft 15 (Fig. 3) in which there may be arranged stairs and/or a hoisting means (not shown) for controlling personnel for execution of operational control, at least during the towing and installation phases of the platform. The columns 12 are shown to be vertical, but they may also be slightly sloping towards each other towards the top.

In Fig. 4 there is shown a second embodiment of the platform according to the invention. The caisson structure 1 is of the same design as in Fig. 1, but the framework structure 16 is different and here consists of a plurality of vertical columns or posts 17 between which there extend a plurality of horizontal struts 18 and oblique struts 19. The framework surrounds a hollow column 20 corresponding to the hollow column 12 in Fig. 1. Thus, the column 20 does not necessarily constitute a part of the actual framework structure, such as is the case in the embodiment in Fig. 1. The hollow column 20 may suitably be dimensioned to withstand the ambient loads on the site of installation only when it is connected to and supported by the framework, but to have sufficient strength to withstand the ambient loads when it stands unbraced in sheltered waters. The hollow column 20 is here shown centric inside of the panels of the framework structure. This is only an example. The hollow column may be placed at any place inside of the panels, it may be placed outside of the panels or it may be a part of the framework structure. The embodiment in Fig. 4 is schematically shown to include a number of casings or conductor pipes extending outside of the caisson and having an oblique course in the area of penetration into the sea bed. A first type of tube 5 21 goes straightly down and a possible deviation from a vertical line is made beneath the sea bed. Another type of tube 22 gets a deviation before the sea bed, but is inside of the caisson. A third type of tube 23 gets a deviation high up and is carried at the outside of the caisson, and

10 possibly at the outside of the base before the tube penetrates the sea bed. It may also be deflected along the caisson - thereby it is possible to obtain a largest .• possible deviation of the casing when it reaches the oil or gas zone. This may be of considerable economic value.

15 Especially at the uppermost levels of the platform risers and conductors will incur substantial drag forces, not least because of the fact that individually they will be able to get a considerable sea-weed growth, and it may be favourable to transfer a part thereof to mass forces.

20 All or some of the tubes may be surrounded by a jacket forming a larger, closed volume, and the wave forces will even be able to reduce the mass forces totally. Simultaneously, this volume may be able to form a safety volume and cause the interconnection with the deck in

25. floating condition to be carried out under safer circumstances if it is placed at a sufficiently high level. This is schematically shown at 24 in Fig. 4.

Such as subsequently further described, the framework 16 and the caisson 1 may be built separate frαrt each other

30 and be completely or partly finished inclusive of mechanical equipment and thereafter be assembled in accordance with the method according to the invention. In the embodiment shown in Fig. 4, the framework is then floated in place above the caisson when this is in a suitably submerged

35 position. The framework may then be built with a side opening for introduction of the column 20, as suggested at 25 in Fig. 5. In this case it may be topical to post- assemble a limited number of struts 26 and 27, e.g. as shown in Fig . 5.

In the embodiment in Fig. 6 the framework structure 16 is carried out in the same manner as in Fig. 4, but the caisson 28 is different from the embodiment in Figs. 1 and 4. In this embodiment the caisson comprises several oblique, upwards converging cells 29 carried upwards from a base 30 without the cells having any mutual connection between the upper side of the base and the uppermost portion of the caisson where the cells 29 are cast together. A skirt 31 depends from the base in order to penetrate the sea bed on the site of installation. In such a monolithic embodiment it is unnecessary with a stiffening framework structure between the cells, something which is advantageous as such a structure is expensive and moreover delays the building. Such as in Fig. 4, in Fig. 6 there are also suggested casings 32 extending at the outside of the caisson, and also at the outside of the framework structure.

As mentioned above, the number of cells in the caisson and the mutual arrangement thereof may vary. In Figs. 7 and 8 there is shown a platform embodiment wherein the caisson is formed by only one cell 33 having a circular cross-section.

In Fig. 9 there is shown a fifth embodiment of the platform according to the invention consisting of a caisson 34 and a framework structure 35. A strong column 36 is ° cantilevered frc the caisson structure. To this column 36 there is braced a lighter structure of columns 37. It is the strong column 36 which braces the total structure above the caisson 34. The arrangement of the columns 37 of the framework with associated braces or struts 38, 39 appears ° from the cross-section in Fig. 10.

In Fig. 9 there is shown an access shaft 40 extending downwards through the caisson 34, so that dry access is established all the way from the deck 41 via the hollow column 36 and down to the sea bed 42. At the bottom the ⁵ access shaft 40 passes into a chamber 43 which may be used for different technical devices which are necessary to establish the platform, 1.7

for the operation of the platform, and also to pull in tubes from the outside, something which takes place by way of a chanel 44.

The platform in Fig. 9 is shown to be fixed to the sea bed by means of piles 45. Such piling may be necessary, e.g. to reduce settlings.

As far as the framework structure is concerned, it is obvious that this may be shaped in many different ways. For example, the lowermost part of the framework structure may be made of strong columns with braces. The columns may be cast in concrete or made in steel. The uppermost part of the framework structure may be a lighter structure in steel which is mounted on top of the former. The lowermost part may also be conical columns without braces. In a suitable embodiment the upper framework structure may consist of four supporting legs connected to oblique plates arranged such that each of them intersects a pair of other oblique plates. This enables extra many connections which transfer horizontal force, so that the lifetime of the platform increases. The outer legs as well as the braces are stepped downwards. In order to limit the material thickness in the supporting legs, inner legs are constructed which are pulled into the outer ones. The inner and outer legs are adapted in relation to each other so that there is a little distance therebetween. Connection is obtained by injection of mortar (grouting) .

Fig. 11 shows a sixth embodiment of the platform according to the invention and illustrates a special arrangement of the lower concrete structure. The base here consists of a pair of concentric rings of totally eighteen domes 46 at bottom level. In the illustrated case a central compartment 47 is provided to give room for pre-drilled wells 48. If such a compartment is not to be provided, the central opening will be covered by an additional dome 46. At least twelve outermost domes and the central dome (when this is present) are supported by skirts 49 penetrating the sea bed. All domes or a selection thereof may be covered with flat discs. These discs may be prestressed in a simple manner so that tensile stresses resulting in cracks are avoided. The seven inner domes are covered by cells 50 up to another level of domes 51. Inclined planes 52 extend from the periphery of the twelve outermost cells up to the latter dome level. The seven inner cells continue further upwards from this dome level in the form of* cells 53 forming the caisson structure 54 of the platform. At the top, the cells 53 are terminated•with domes 55. The stated solution renders an inner floating volume to these surfaces during the first floating phases. Totally, the solution provides a relatively light base which to a great extent can be finished in dock with limited towing draft out to the deep water location for further construction.

As shown in Fig. 11, the walls in the cells of the caisson may have a decreasing thickness upwards from a certain level. This is favourable for i.a. the floating stability and is made possible as a result of the fact that the hydrostatic pressure, which is dimensioning, decreases. In slip forming this will require a special slip form system with variable bents which are previously not known on structures having several connected cells.

Said compartment 47 which is arranged to give room for pre-drilled wells rising above the sea bed, is in principle formed by raising the bottom of the base structure in a suitable region. The pre-drilled wells are drilled from a drillingrig before the platform is installed. Thereafter the well tubes will be extended to deck level.

In the embodiment of the Fig. 11 there is shown an access shaft in the form of a hollow cylinder 56 extending downwards through the framework structure 57 as well as the caisson 54. A special solution for a part of the mechanical systems is conceivable in that these are finished in a container which is subsequently transported in position so that it can be lifted up to a dome and then be placed there. This container may then form an extension of the inner, hollow cylinder. Such a container 58 is shown in Fig. 11. The container 58 with mechanical systems is connected to a riser tunnel 59 which may consist of a large steel tube or concrete tube and is pulled into the base structure.

The cells 53 above the base structure may consist of circular cells which are tangent to each other. As shown in Fig. 12, the cells may alternatively consist of elements

60 which are circular along the outer periphery and in the 5 interior have straight walls 61 forming a honeycomb-shaped, hexagonal geometry in the interior.

The connection between the upper framework structure and the lower concrete structure may for example be obtained _ as shown in Fig. 13, in that the framework structure is terminated by a flat bottom plate 62 going beyond the cylinder shape of the legs 63. The plates 62 are placed on concrete columns 64 cast around the cylinder walls on the concrete structure 65. Connection between the two structures is _S obtained by prestressing, e.g. prestressed cables 66. Braces transferring horizontal forces down into the concrete structure must be fixed by a similar method.

In the following there will be described various embodiments of the method of manufacturing the platform 0 according to the invention, and especially for interconnection of the framework structure and caisson of the platform.

In the embodiment according to Figs. 14-16, the frame¬ work structure 70 with cantilevered auxiliary structure 71 is finished in upright position on a separate place. It is 5 thereafter brought onto floating bodies 72 as shown in Fig. 15 and is towed in over the ballasted caisson 73 as shown in Fig. 16 and is interconnected (mated) therewith in that the caisson is deballasted. Thereafter the framework structure may be securely fixed to the caisson. 0 In the platform embodiment shown in Fig. 17 there is established a divided column for use in the interconnection of the framework structure and the caisson. The platform embodiment corresponds to the embodiment in Fig. 6. It appears from Fig. 17 that the hollow column 20 in Fig. 6 in 5 the construction phase is divided into an upper column part 20a arranged in the upper portion of the framework structure 16, and a lower column part 20b projecting upwards from the caisson 28. However, the hollow, air-filled column does not need to be divided in the shown manner.

After the hollow column 20b is built on the caisson 28, this is submerged as shown in Fig. 17 so that only a small part of the column rises above the water surface 4. The separately built framework structure 16 with the column part 20a is floated in vertical position with submerged lower end to the position of the caisson and is mated therewith, the operation being controlled by means of controlling personnel in the hollow column. As shown, the framework structure is suitably provided with extra buoyancy tanks 74 for keeping the structure floating and for controlling the lowering of the framework to a suitable depth in relation to the top of the caisson. After the fitting-together, the caisson is deballasted for raising of the entire framework structure to a position above the water surface, whereafter the framework structure is secured to the caisson and the hollow column in free air, and also the upper and lower column parts are interconnected in a tight manner. By raising the entire framework structure to free air, complicated underwater assembly operations by means of divers are avoided.

Figs. 18-21 show different phases of a third embodiment of the method for establishing the framework structure on the caisson. Firstly, the framework structure 75 is built horizontally in a known manner. Thereafter it is set afloat and ballasted until it is vertical. Hoisting means 76 are attached on floating bodies 77 or barges to the framework structure 75, and this is either raised up above the water or raised to a small draft. Thereafter the framework structure 75 is brought in above the down-ballasted caisson 78 and interconnection is effected. If the framework structure is not quite free of the water surface, the caisson must have parts which project upwards and which do not inter¬ fere with the framework structure. This is suggested in Fig. 21 wherein an extra safety volume 79 is built above the upper domes of the caisson. By raising the entire framework structure to free air, complicated underwater assembly operations by means of divers are avoided. The framework structure and the caisson will be equipped with special means for achieving a safe securing of these platform parts to each other. These means may be of various types, e.g. such as described in connection with Fig. 13.

Said extra safety volume 79 on the caisson 78 may be established in different manners, e.g. by casting a cofferdam. If this cofferdam interferes with the framework, one defers introducing some of the lowermost rods in the framework until after assembly. The assembly itself takes place in that the framework is floated over the concrete structure. If convenient, the framework is kept stationary and the concrete structure is moved. The concrete structure is controlled via the safety volume or a separate control column.

As an alternative in connection with the above- mentioned method, thewhole caisson may be lowered .under water before interconnection with the framework. The caisson may be stabilized by use of temporary buoyancy bodies, or by use of crane equipment. For safety reasons it may be conceivable that the caisson is deballasted on a place where there will be a small bottom clearance.

In Figs. 22-24 there is shown an embodiment wherein there is used a temporary articulation means 80 as a connection between the concrete structure 81 and the framework structure 82 which in turn is lying in floating condition on a barge 83. The concrete part is submerged under water, and the framework on the barge then rises progressively because of the buoyancy. Final assembly is carried out after the structure is deballasted so that the articulation comes into free air. The structure is secured against getting lost in that it is taken care that the sea bed is at a suitable level. It will be clear that this method may also be used in connection with a concrete caisson including a column projecting therefrom as shown in Figs. 6 and 17.

Fig. 25-27 show an embodiment wherein the caisson con¬ struction 84, i.e. the concrete part, is caused to assume a floating position deviating maximum 90 from vertical position of the caisson, so that the concrete part 84 and the framework part 85 in corresponding floating position can be coupled together before the entire structure is again erected by ballasting. Fig. 28-30 shows an embodiment wherein the caisson structure 86 is ballasted to an oblique floating position and is brought into position adjacent to the lower end of the framework structure 87 floating in a horizontal (or suitably inclined) position on a floating means 88. The caisson and framework are thereafter interconnected by means of an articulation means 89 at the lower, adjacent end portions, whereafter the two parts 86, 87 by means of suitable ballast and possibly also buoyancy means are caused to assume the corresponding oblique position shown in Fig.29, so that provisional or partial interconnection can be effected. After the provisional interconnection,the whole structure is ballasted so that it is brought to upright position, whereafter final interconnection is effected. As a variant of the last-mentioned embodiment, the framework part may be brought to an oblique floating position, corresponding to that of the concrete part by means of suitable floating or buoyancy bodies, before the two parts are moved together and interconnected provisionally to each other and the entire structure thereafter is ballasted to an upright position. Said articulation between the concrete part and the framework part may then be omitted.

Fig. 31-32 illustrate an embodiment wherein the framework structure is built in an upright position in two or more parts 90,91 and transported by barges 92 for inter- connection with the caisson structure 93 which is in a suitably submerged floating position. Final interconnection of the different parts can be effected in free air, either by welding or by injection of mortar (grouting) .

In connection with the placing and assembly of the finished framework structure (or parts thereof) on the caisson structure, the caisson may be provided with temporary auxiliary bodies mounted thereon and projecting upwards from the upper end of the caisson. These bodies are then utilized in ballasting-down of the caisson, so that this in a safe manner can be lowered down a suitable distance beneath the water surface. It is thereby achieved that the framework structure in a simple and safe manner can be brought in over the caisson structure, the framework being possibly partly submerged in a stable position on suitable floating means.

As regards the manufacture of the framework structure, one or more columns may be cast in concrete by slip forming in embodiments corresponding to the embodiment in Fig. 1, the columns being cast directly on the caisson structure. The columns are mutually braced by means of braces in steel or concrete. It may be suitable to ballast the platform to a greater draft when the bracing is mounted. The framework structure may also be built in that e.g. four panels are transported separately on barges and coupled to the concrete part by means of a hinge. The concrete part is gradually submerged, and the panels, which are connected to the barges, are pivoted to an upright position. The framework may then consist of eight supporting columns which are connected in pairs. Alternatively, the barges may be replaced by containers which are subsequently used as an integrated part of the deck structure of the platform. In this method there is built up a column projecting from the caisson and which is used as a support for lines pulling the framework panels up to a vertical position. The framework panels may possibly also be placed in correct position on the caisson by means of lifting means. After the framework structure and the caisson have been secured to each other, the platform is submerged to a position with a little freeboard, whereafter the deck structure 5 of the platform (see Figs. 1 and 4) is placed on the top of the framework structure by fitting-together in a suitable manner. The weight of the deck is transferred to the platform by deballasting of the caisson.

Claims

P a t e n t C l a i m s

1. A gravity platform for a great water depth, wherein the platform in operational condition is intended to stand stably on the sea bed (3) essentially by its own weight and in this position is intended to project above the water surface (4) for support of a deck structure (5), the platform comprising a lower part in the form of a base

(6, 7) and a caisson structure (1 ; 28) of concrete, and an upper part in the form of a framework structure (2; 16) carried by the caisson structure, c h a r a c t e r i z e d in that the framework structure (1 ; 28) as well as the caisson structure (2; 16) has a height of at least 90 , and that the caisson structure (1; 28) is a monolithic structure of such a great height that the centre of gravity of the platform is located below its centre of buoyancy- in a floa¬ ting position during towing-out at a draft which is essentially equal to or less than the total height of the caisson structure (1; 28), so that the platform is floating- stable without external means.

2. A plat orm according to claim 1, c h a r a c ¬ t e r i z e d in that at least one hollow, air-filled column (12_f 20) is provided in connection with the frame¬ work (2; 16), which column extends at least between the top of the framework structure (2; 16) and the top of the caisson structure (1; 28) and provides at least temporary, dry access to the lower part of the platform, and which is arranged for execution of operational control during diffe¬ rent phases, such as assembly, towing, installation and operation of the platform.

3. A platform according to claim 2, c h a r a c t e r i z e d in that the hollow column (12; 20) also extends downwards in the caisson structure (1 ; 28) and provides dry access almost or all the way down to the sea bed.

4. A platform according to one of the claims 1 - 3, c h a r a c t e r i z e d in that it includes a bottom structure (46, 50, 51, 52) of which a part is raised so that 25

there is formed a compartment (47) for predrilled wells (48) above which the platform may be installed.

5. A platform according to any of the preceding claims, c h a r a c t e r i z e d in that, in the lower part of the caisson structure (33) there is provided a dry compartment (43) having dry access and being arranged for pulling-in of tubes.

6. A platform according to any of the preceding claims, c h a r a c t e r i z e d in that, in the region between the caisson structure (34) and the sea bed (42) there is provided a spaceous, inclined tube (44) for the reception and guidance of conductor tubes.

7. A platform according to any of the preceding claims, c h a r a c t e r i z e d in that it includes a number of casings (22, 23) extending on the outside of the caisson structure (1) and having an oblique course in the region of penetration into the sea bed (3) .

8. A platform according to any of the preceding claims, wherein the caisson structure (1; 54) is constructed from a plurality of upright cells (9; 53), c h a r a c ¬ t e r i z e d in that the outer cell walls in cross- section consist of circle sectors (60) , whereas the inner cell walls (61) at least from a certain height level consist of straight walls in hexagonal shape which may be solid or show cavities in the adjacent joints.

9. A platform according to any of the preceding claims, c h a r a c t e r i z e d in that the caisson structure (54) is constructed from a plurality of upright cells (53) wherein the cell walls have a decreasing thick¬ ness in upwards direction.

10. A platform according to any of the preceding claims, c h a r a c t e r i z e d in that the framework structure comprises a number of main columns consisting of steel tubes having stepped diameter in upwards direction and surrounding inner, correspondingly stepped steel columns.

11. A platform according to any of the preceding claims, c h a r a c t e r i z e d in that the framework structure (16) in the region above the final water level on the installation site is supplemented with a floating volume (24) forming a safety volume during e.g. inter¬ connection with the platform deck.

12. A method of manufacturing a gravity platform for a great water depth, wherein the platform in operational condition is intended to stand stably on the sea bed (3) essentially by its own weight and in this position is in¬ tended to project above the water surface (4) for support of a deck structure (5), the platform comprising a lower part in the form of a base (6, 7) and a caisson structure

(1; 28) of concrete, and an upper part in the form of a framework structure (2; 16) carried by the caisson structure, c h a r a c t e r i z e d in that the framework structure

(2; 16) and the caisson structure (1; 28) are built sepa¬ rately at respective sites, and that at least a part of the framework structure as an essentially finished unit in a floating position is brought into position to^* be connected to the caisson structure which is in a suitably submerged floating position for said interconnection.

13. A method according to claim 12, c h a r a c ¬ t e r i z e d in that at least a part of the framework structure (70) is essentially finished in upright position on the building site and is thereafter brought to a floating position suspended or supported between floating bodies

(72) , and in this position is interconnected with the sub¬ merged, vertical caisson structure (73) .

14. A method according to claim 12, c h a r a c ¬ t e r i z e d in that at least a part of the framework structure (75) is built in horizontal position, is set afloat, is ballasted to essentially vertical position, is hoisted up between floating bodies (77) so that it is clear or almost clear of the water surface, and is brought in over the submerged vertical caisson structure (78) and connected thereto.

15. A method according to claim 13 or 14, c h a r a c t e r i z e d in that the framework structure is built in two or more parts (90, 91) before the inter¬ connection with the caisson structure (93) . and that the parts are coupled to the caisson structure successively after each other.

16. A method according to claim 12, c h a r a c ¬ t e r i z e d in that the framework structure (82) is brought in floating, essentially horizontal position ahead to the caisson structure (81) which is ballasted to a sub¬ merged, essentially vertical position and is attached to the caisson structure by an articulation means (80), where¬ after the caisson structure (81) is further submerged so that the framework structure (82) thereby is brought to a vertical position during simultaneous influence of suitable lifting means (83), such as floating bodies, cranes, stays or the like.

17. A method according to claim 12, c h a r a c¬ t e r i z e d in that the caisson structure (84) in its submerged floating position is ballasted to a floating position deviating maximum 90° from vertical position of the caisson, that the framework structure (85) is brought up to the caisson structure (84) in a correspondingly in¬ clined or horizontal position and is coupled to the caisson structure, and that the latter is thereafter ballasted so that the platform as a whole is brought to an upright position.

18. A method according to claim 12, c h a r a c ¬ t e r i z e d in that the caisson structure (86) is ballasted to an oblique floating position and is brought into position adjacent to the lower end of the framework structure (87) , this being floating in an essentially horizontal position, that the caisson (86) and the frame¬ work (87) are interconnected by means of an articulation means (89) at the lower, adjacent end portions, and that the two parts (86, 87) are caused to assume a correspon¬ ding oblique position and thereafter are provisionally interconnected to each other, whereafter the entire struc¬ ture is ballasted so that it is brought to an upright posi¬ tion.

19. A method according to claim 12, c h a r a c¬ t e r i z e d in that the caisson structure as well as the framework structure are caused to assume correspondingly oblique floating positions by means of ballasting and/or buoyancy means, and that the two parts are thereafter brought together and interconnected provisionally with each other, whereafter the entire structure is ballasted so that it is brought to an upright position.

20. Use of a gravity platform which in operational condition is intended to stand stably on the sea bed (3) essentially by its own weight and in this position is intended to project above the water surface (4) for support of a deck structure (5) , the platform comprising a lower part in the form of a base (6, 7) and a caisson structure

(1 ; 28) of concrete, and an upper part in the form of a framework structure (2; 1 ) carried by the caisson structure, at a water depth which is so great that the wave action in the depth region in which the caisson extends is- unessential in relation to the wave action in the depth region in which the framework structure extends, the caisson still being so high that it contributes to a quite substantial portion of the floating stability of the platform during towing thereof without external stabilizing means.

21. Use of a gravity platform according to claim 20 at an operational site where there is a limited towing draft on the towing route from the building site of the platform to the operational site.