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Title:
A FRICTION CLAMP FOR TRANSFER OF AN OFFSHORE PLATFORM TOPSIDES FROM A SUBSTRUCTURE TO A FLOATING TRANSPORTER
Document Type and Number:
WIPO Patent Application WO/1999/043542
Kind Code:
A1
Abstract:
In a method for the transfer of an offshore platform topsides (3) from a substructure (2) fixed to the seabed to a floating transporter (1), where the transporter (1) is lowered by means of ballasting and introduced under the platform topsides (3), whereupon the transporter is deballasted thus causing it to be raised and lifting the topsides from the substructure, a friction clamp (7) is employed comprising friction portions (8) adapted for clamping round the leg (4) along a length (1) of the leg, and a lifting portion (14) adapted for abutment against the transporter (1) for lifting in the vertical direction.

Inventors:
Haugsøen, Per Bull (Gyldenløvesgate 12 Oslo, N-0260, NO)
Foss, Gunnar (Mezenplein 11 ZS Denhaag, NL-2566, NL)
Application Number:
PCT/NO1999/000062
Publication Date:
September 02, 1999
Filing Date:
February 25, 1999
Export Citation:
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Assignee:
Marine, Shuttle Operations AS. (P.O.Box 1137 Sandnes, N-4391, NO)
Haugsøen, Per Bull (Gyldenløvesgate 12 Oslo, N-0260, NO)
Foss, Gunnar (Mezenplein 11 ZS Denhaag, NL-2566, NL)
International Classes:
B63B9/06; E02B17/00; (IPC1-7): B63B35/00; E02B17/00
Domestic Patent References:
1998-06-25
1998-06-25
1999-02-11
Foreign References:
US3033525A1962-05-08
US2907172A1959-10-06
Attorney, Agent or Firm:
Onsagers, Patentkontor Defensor -. AS. (P.O. Box 265 Sentrum Oslo, N-0103, NO)
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Claims:
PATENT CLAIMS
1. A method for transfer of an offshore platform topsides (3) from a substructure (2) fixed to the seabed to a floating transporter (1), wherein the substructure (2) at least in an upper portion which is connected to the topsides (3) is composed of tubular legs (4), where the transporter (1) is lowered by means of ballasting and moved in under the platform topsides (3), whereupon the transporter (1) is deballasted so that it is raised and lifts the topsides from the substructure, characterized in that it comprises the following steps: a) the legs (4) are cut (9) and a friction clamp (7) with oblong friction portions (8) is provided round each of the legs (4) and is clamped into abutment against the legs on the top (10) and the underside (11) of the cuts (9), the friction clamps' (7) total frictional force above the cuts (9) being arranged to be greater than the weight of the topsides (3), and the friction clamps'total frictional force below the cuts being arranged to be equal to the buoyancy of the transporter (1) required in order to lift the topsides (3) to a height where the risk of collision between opposing cut surfaces (12,13) of the cutoff legs (4) due to vertical movement of the transporter (1) and the topsides (3) is not present, this required buoyancy being less than the weight of the topsides, b) the transporter (1) is lowered by means of ballasting and moved in under the platform topsides (3) in a known per se manner, c) the transporter is raised by means of deballasting, and brought into abutment against lifting portions (14) on the friction clamps (7), d) the transporter is further deballasted, whereby, when the transporter's (1) buoyancy is equal to the sum of the weight of the topsides (3) and the frictional force between the friction clamps (7) and the legs (4) on the underside (11) of the cuts (9), the friction clamps are released on the underside of the cuts and the topsides (3) is lifted up from the substructure (2).
2. A method according to claim 1, characterized in that vertically acting spring devices (15) are arranged between the transporter (1) and the friction clamps'lifting portions (14) under step c).
3. A method according to claim 1 or 2, characterized in that spring devices (16) which are horizontally acting and radially acting relative to the legs are provided between the transporter (1) and the friction clamps (7) under step c).
4. A method according to claim 2 or 3, characterized in that horizontal sliding surfaces (17) under step c) are provided between the transporter (1) and the friction clamps'lifting portions (14), in series with the vertically acting spring devices (15).
5. A method according to one of the preceding claims, characterized in that the frictional force between portions of the legs (4) below the cuts (9) and corresponding portions of the friction clamps (7) is reduced under step d).
6. A device for use in the transfer of an offshore platform topsides (3) from a substructure (2) fixed to the seabed to a floating transporter (1), according to one of the preceding claims, characterized in that it comprises a friction clamp (7) with friction portions (8) adapted to clamping round the leg (4) along a length (1) of the leg, and a lifting portion (14) adapted to abutment against the transporter (1) for lifting in the vertical direction.
7. A device according to claim 6, characterized in that the friction portions' (8) clamping round the leg is arranged differently adjustably along the said length (1) of the leg (4).
8. A device according to claim 6 or 7, characterized in that it comprises a vertically acting spring device (15) between the friction clamps'lifting portion (14) and the transporter (1).
9. A device according to one of the claims 6 to 8, characterized in that it comprises spring devices (16) radially acting relative to the leg (4) between the friction clamp (7) and the transporter (1).
10. A device according to claim 8 or 9, characterized in that the spring devices (15,16) comprise alternately arranged lamellae of steel (18,20) and an elastomer (19,21).
Description:
A friction clamp for transfer of an offshore platform topsides from a substructure to a floating transporter The invention relates to a method and a device for transfer of an offshore platform topsides from a substructure fixed to the seabed to a floating transporter.

The transfer of offshore platform topsides from a substructure fixed to the seabed to a floating transporter occurs in connection with the removal of offshore platforms.

Norwegian patent no. 160 424 describes a devices which is composed of two parallel groups of floats. Each group comprises a horizontal float to which vertical floats are attached at one of their ends, while their other ends are attached to longitudinal beams. The end of one group is connected to the end of the other by two horizontal, longitudinal floats and a box beam. The longitudinal beams bear movable support beams which can be moved in under the cargo which has to be lifted and transported.

GB 2 165 188 A describes a vessel for installation and removal of a deck module on to or from a platform substructure. The vessel is U-shaped with an opening where the platform substructure is located, and comprises a system of movable pillars on each side of the opening for support and positioning of the module.

Norwegian patent application no. 973562 describes a transporter for removal of an offshore platform topsides from an associated substructure, which transporter consists of an oblong, ballastable structure with a U-shaped cross section and a prismatic enveloping surface, and comprises two long sides, an intermediate underside and an opposing open top. At one end of the underside there is a recess which can accommodate the substructure when the transporter is located with the underside horizontally down in the water, and on the side of the recess the long sides have abutment portions which, when the transporter is deballasted, can be brought into abutment against the platform topsides and lift it, thus transferring it to the transporter.

It is also conceivable to transfer a platform topsides from a substructure by moving one or two ballastable barges in under the topsides and deballasting the barges so that they raise the topsides.

When an offshore platform topsides is transferred from a substructure fixed to the seabed to a floating transporter, due to the tremendous weight of such a platform topsides, all the elements involved in the transfer are exposed to enormous forces. The design of these elements is therefore a field to which great importance should be attached, and in which the solutions are not obvious.

A common problem when transferring objects at sea is impacts due to the movement of the sea. When transferring an offshore platform topsides from a substructure fixed to the seabed to a floating transporter, the substructure will substantially be at rest, while the transporter will move dependent on the movement of the sea. When the transporter is located close to the topsides and is in the process of being connected thereto, its movement will be capable of causing jarring impacts to occur between the transporter and the topsides, which, if no allowance is made for them, will be capable of damaging both the transporter and the topsides. A similar problem exists after the topsides has been secured to the transporter and is in the process of being released from the substructure, when, due to movement caused by the transporter, the topsides might collide with the substructure.

A further problem which is manifested in all offshore structures is oscillations generated in the structure due to wave action. In order to avoid this all offshore structures must be designed on the basis that their natural frequency must be outside the range of the waves'excitation frequency. After attachment of the transporter to the topsides as described in Norwegian patent application no. 973562, on account of the rigidity of the attachment to the topsides, the transporter will obtain a completely different natural frequency to that which it had before attachment. If this is not taken into account, the transporter may begin to oscillate due to excitation from the waves, which may bring it out of control, resulting in jarring impacts between the transporter and the topsides.

Thus the transfer of an offshore platform topsides from a substructure fixed to the seabed to a floating transporter as described in the above-mentioned publications is not without its problems.

The object of the invention is to provide methods and devices for transfer of an offshore platform topsides from a substructure fixed to the seabed to a

floating transporter, which methods and devices should not be encumbered by the above-mentioned problems.

Thus it is an object to provide an indication of the design of elements for the transfer and a method for their application wherein the problems are solved of impacts between the transporter and the deck when the transporter is in the process of being brought into abutment against the topsides and impacts between the topsides and the substructure when the topsides is in the process of being released from the substructure. A further object is to solve the problem of oscillations in the transporter after attachment to the topsides due to excitation from the waves. Further, more detailed objects will appear in the special part of the description.

The objects are achieved with a method and a device of the type mentioned in the introduction, characterized by the features which are indicated in the claims.

In the detailed description the invention is explained in connection with a transporter of the type which is described in Norwegian patent application no. 973562. It should be understood, however, that the invention will also be able to be employed together with other transporters, such as barges.

The invention will now be explained in more detail in connection with a description of individual specific embodiments, and with reference to the drawing, in which: fig. 1 is a perspective view of a transporter which is located in the water beside an offshore installation fixed to the seabed.

Fig. 2 is a curve illustrating the problem of oscillations of the transporter due to the wave action.

Figs. 3-6 illustrate a transporter provided with beams according to the invention during various stages of transfer of a platform topsides to the transporter according to the invention.

Fig. 7 illustrates lifting beams and fenders for use in the transfer of the platform topsides.

Fig. 8 illustrates the lifting beams in a position where they can lift the topsides.

Fig. 9 illustrates a lifting beam in closer detail.

Fig. 10 illustrates a friction clamp for use in the transfer of the platform topsides.

Figs. 11-13 illustrate the juxtaposition of the lifting beam, the friction clamp and a cut platform leg during the transfer of the platform topsides.

Fig. 14 illustrates a pair of lifting beams which together form a uniform lifting beam for lifting the friction clamp.

Fig. 15 illustrates the lifting beam and the friction clamp after the platform leg which has been cut below has been released from the friction clamp.

Fig. 1 illustrates a floating transporter 1 which is located in the water beside an offshore structure consisting of a platform topsides 3 resting on a substructure 2 which is located on the seabed 6. The transporter consists of an underside 25 and a long side 24 arranged on each side of the underside, the three sides thereby forming an oblong structure with a U-shaped cross section. The transporter is shown in a deballasted position where its underside is located on the surface of the sea 5, and the figure illustrates how the end of the underside which is facing the platform topsides, hereinafter called the front end, has a recess 26. The outermost part of the recess, which is located at the end of the transporter, is called the recess's opening 27.

On transfer of the topsides 3 from the substructure 2 to the transporter 1, the transporter is lowered by means of ballasting. The front end of the transporter is then introduced under the platform topsides, the substructure thereby entering the recess's opening 27, and subsequently the actual recess 26, while at the same time the long sides 24 remain under/beside the topsides 3. The transporter 1 is then deballasted, thus causing it to be raised and lifting the topsides 3 from the substructure 2, whereupon the transporter with the topsides are removed from the substructure.

Fig. 2 shows a curve illustrating the problem mentioned at the beginning of oscillations of the transporter due to excitation from the wave action. The curve shows amplitude x for the transporter's horizontal oscillations after attachment to the topsides as a function of the horizontal rigidity k in the attachment. Fig. 2 gives typical values for a transporter and a large topsides for a platform in the North Sea.

Wave action is a highly complicated phenomenon, with different amplitudes, directions and frequencies. Analyses show that the excitation from the waves on the transporter which has the greatest significance has a period of round 8 seconds, designated first order movement. First order movement has relatively little amplitude, but great forces are required to suppress it. The excitation with the second greatest significance has a period of round 100 seconds, and is designated second order movement. Second order movement has large amplitude, but can be suppressed with relatively small forces.

Further excitations also have an influence on the transporter, but are of minor significance.

As illustrated in the figure the amplitude has a low value at 0 rigidity, i. e. no attachment to the topsides. The transporter's amplitude here corresponds substantially to first order movement.

In area A the amplitude increases with increasing rigidity. The rigidity here is so slight that the transporter's movement will be capable of causing it to collide with the topsides. Consequently any rigidity in this area may be problematic.

In area B the amplitude has a maximum point due to the second order movement, and the amplitude here is so great that any attempt at attachment in this area will be highly problematic.

In area C the rigidity is so great that it suppresses the second order movement, while the first order movement is allowed to take place without much suppression. The amplitude here corresponds substantially to amplitude of the first order movement, while at the same time the forces between the transporter and the topsides are relatively insignificant. Both the movement and the forces here are so slight that attachment can be implemented.

In area D the amplitude has a maximum point due to resonance with the first order movement, and here both the amplitude and the forces are so great that attachment may be problematic.

In area E both the second order movement and the first order movement are suppressed. The amplitude here is small, but the forces in the attachment are so great that attachment may be highly problematic.

Consequently, the preferred operational area for attachment is area C.

In addition to the ability to counteract the transporter's oscillatory movement by establishing a desired rigidity, the oscillatory movements may also be counteracted with damping devices which absorb the oscillatory movement by means of friction, as well as reducing their velocity. Such damping devices can be established by mooring the transporter to the seabed with oscillation-damping moorings in the form of chains or wires. It is also possible to establish a cross between a rigidity and a damping attachment by using such moorings between the transporter and the topsides, or between the transporter and the substructure. A substantial part of the transporter's oscillatory movement, however, has to be established by a rigidity in the actual attachment of the transporter to the topsides, since otherwise the topsides might work loose from the transporter.

A description will now be given of different aspects of the invention which are intended to establish this rigidity between the transporter and the topsides.

Fig. 3 illustrates a transporter provided with devices according to the invention, ballasted in such a manner that it is located with the underside 25 down in the water, and the long sides 24 projecting up from the surface 5 of the sea. At a level above the surface, on the illustrated embodiment near the top of the long sides 24, the transporter comprises a front cross beam 28 which extends between the transporter's long sides 24. The front cross beam 28 is located over the recess's opening 27, but since the underside 25 with the recess 26 are located under water, the recess's opening is not illustrated in fig. 3. We refer therefore to fig. 1, which shows the position of the recess's opening 27. The transporter further comprises two rear cross beams 33 which extend between the long sides 24 on the opposite side of the recess 26 relative to the recess's opening 27. The front cross beam 28 and the rear cross beams 33 rest on brackets or mountings 35 and 36 respectively attached to the long sides 24.

The transporter further comprises longitudinal beams 30,30'which are supported by the cross beams 28,33. The longitudinal beams 30,30'are arranged in pairs at intervals which are slightly larger than the diameter of the substructure's legs, and are bounded between longitudinal openings 29 for insertion of the substructure's legs, which will be described in more detail

with reference to figs. 5 and 6. Fig. 3 also illustrates lifting beams 31, 31', which will be described in more detail with reference to fig. 7 and beyond.

The implementation of the method according to the invention will now be described together with a description of the devices according to the invention.

Fig. 4 illustrates the front cross beam 28 in a position where, before the actual transfer of the topsides, it is not located over the recess's opening, thus providing access to the area over the recess 26. This is achieved by having the front cross beam movably mounted in its longitudinal direction PI in one of the mountings 35 in one of the long sides 24. It should be understood, however, that this lateral movement of the front cross beam 28 is only one of several possible ways of opening access to the area over the recess 26, and this can also be achieved by attaching one end of the front cross beam 28 to the mounting 35 by a rotating joint, or lifting away the front cross beam with a crane, which may, for example, be located on an auxiliary vessel or the platform topsides.

When the front cross beam 28 is not located over the recess's opening, and thereby does not constitute a support for the longitudinal beams 30,30', the longitudinal beams are supported only by the rear cross beams 33, which due to the weight of the longitudinal beams will be exposed to a moment. This is an important reason for having two rear cross beams 33, since the moment is thereby absorbed as a force couple, but of course it is also possible to employ other designs of the rear cross beams, such as a torsionally rigid beam which is capable of absorbing the moment.

The platform topsides and the platform substructure may be designed in a number of different ways, which are of no importance for the invention. A common design of the substructure, however, involves a substructure which is composed of tubular legs, at least in an upper portion which is connected to the topsides.

Before the actual transfer of the topsides according to the invention these tubular legs are cut and a clamp is affixed which temporarily secures the topsides to the substructure and holds together the cut surfaces through the legs. These clamps may be of different types, and may have different types of remote control for release. In one type which may be employed securing is

accomplished by means of screws, and the release of the securing is accomplished by blowing off the screws with explosive bodies. A second embodiment of such a clamp is mentioned with reference to fig. 10 and beyond.

After access has been provided to the area over the recess as described with reference to fig. 4, the transporter is moved in under the platform topsides in a known per se manner.

Fig. 5 illustrates the transporter after it has been moved in under the platform topsides. Here the long sides 24 are located on the outside of the substructure, with the result that the substructure is located in the recess 26 (see fig. 1). In order to illustrate the invention the actual topsides is not shown, and the substructure and the recess are located under water, with the result that the only part of the substructure which can be seen is the upper portions of the substructure's legs, indicated by reference numeral 4, with the clamps affixed for temporary securing, indicated by reference numeral 7.

When introduced under the platform topsides the transporter is steered in such a manner that the legs 4 enter the two longitudinal openings 29, as mentioned with reference to fig. 3, the width of the longitudinal openings 29 being slightly larger than the diameter of the legs 4.

The front cross beam 28 is then moved back to its original position over the recess's opening 27, which is illustrated in fig. 6. In this position the front cross beam 28 supports the longitudinal beams 30,30', and since as mentioned the longitudinal beams are also supported by the rear cross beams 33, full support is obtained of the longitudinal beams at both ends.

Fig. 7 illustrates fenders 32 which are arranged on sides of the longitudinal beams 30,30'facing the longitudinal openings 29. These fenders are moved into abutment against the substructure's legs 4, thereby achieving damping of the transporter's 1 horizontal movement in the longitudinal openings' transverse direction P2. In a preferred embodiment the distance between the fenders 32 in the longitudinal openings 29 is less than the diameter of the substructure's legs 4. This results in tight control and immediate damping of the transporter's movement in the transverse direction P2, while at the same time leading to a damping of the transporter's horizontal movement in the longitudinal openings'longitudinal direction P3.

The lifting beams 31,31'for the topsides, which were mentioned with reference to fig. 3, are supported by the longitudinal beams 30,30', and are arranged movably between the position which is illustrated in fig. 3 and fig.

7, where they are located on the side of the longitudinal openings 29, on top of the longitudinal beams 30,30', and a position which is illustrated in fig. 8, where they extend over the longitudinal openings 29, forming a bridge between the longitudinal beams 30,30'.

After the front cross beam has been returned to its original position over the recess's opening, the lifting beams 31, 31'are moved to the position illustrated in fig. 8. The design of the lifting beams must be so adapted to the topsides that they are capable of lifting it.

The transporter is then raised by means of deballasting, and the lifting beams 31, 31'are brought into engagement with the topsides 3. The transporter 1 is further deballasted to a buoyancy which exceeds the weight of the topsides 3, and the clamps 7 for temporary securing of the topsides 3 to the substructure 2 are released, thus causing the topsides to be lifted up from the substructure.

The clamp 7 will hereinafter be described as a friction clamp. However, it is also possible to use other types of clamps for temporary securing of the topsides to the substructure, such as clamps which are screwed directly into the legs on the top and underside of the cuts 9, as illustrated in figs. 10-13.

Fig. 9 illustrates an embodiment of a lifting beam 31 which is provided with a vertical recess 34 which corresponds to the shape of the leg 4 to which a clamp 7 is affixed. The lifting beam's attachment to the longitudinal beams, which, for example, may be composed of horizontal contact surfaces and vertical borings in the lifting beam with corresponding bolts in the longitudinal beams, in order to provide horizontal rotatability and lateral securing, is not shown. A corresponding pair-forming not shown opposing lifting beam 31'has a corresponding vertical recess, and by moving the two lifting beams 31, 31'to a position in which they extend over the longitudinal openings 29, simultaneously being located adjacent to the leg 4 with the two recesses facing each other, the object is achieved that the pair of lifting beams 31,31'forms a uniform double lifting beam which encircles the leg 4.

This double lifting beam is illustrated in fig. 14.

The lifting beam 31 in fig. 9 further comprises horizontally acting spring devices 16 securely attached to the vertical recess 34, and a horizontal sliding surface 17 arranged on the top of the lifting beam, on the side of the vertical recesses 34. The sliding surface 17 is adapted to abut against the topsides 3 to receive the weight of the topsides, while at the same time the topsides is permitted to move in the horizontal direction. The sliding surface 17 may, for example, consist of teflon or an oil film.

Fig. 10 illustrates a section through a friction clamp 7 for holding the leg temporarily together after it has been cut. Together with the friction clamp there is shown a leg 4, which has been cut through at 9. The friction clamp 7 comprises a friction portion 8 which is adapted to be clamped round the leg 4 and abuts against the leg along a length 1 of the leg. The friction clamp's clamping effect is achieved by the fact that the friction portion 8 comprises one or more longitudinal slots, which are located in the section surface in fig.

10, and horizontal screws 37 placed in a row along the leg, across the slots, which when tightened reduce the slot opening, thereby clamping the friction portion 8 against the leg 4. The screws or other tightening devices may be remotely controlled, for example by means of hydraulics. The clamping of the friction portions 8 round the leg is preferably different, i. e. variably adjustably along the length 1 of the leg 4, thus enabling the friction clamp to be adapted to the desired frictional force on the top 10 and the underside 11 of the cut 9.

The friction clamp further comprises a lifting portion 14 adapted to abut against the transporter's lifting beams for lifting the friction clamp with the leg in the vertical direction.

The friction clamp also comprises a vertically acting spring device 15, attached on the underside of the lifting portion 14, to receive vertical forces from vertical movement of the transporter during connection thereto.

Fig. 11 illustrates a section through an intersected leg 4 with a friction clamp 7, encircled by a lifting beam 31. It should be understood that a corresponding, not shown lifting beam 31'encircles the rest of the leg and the friction clamp, with the result that the lifting beams'horizontal sliding surface 17 is located under the whole underside of the vertically acting spring

device 15. This is the position of the lifting beams relative to the friction clamps before deballasting of the transporter.

Fig. 12 illustrates the position during the initial stage of deballasting of the transporter, where the lifting beams'horizontal sliding surfaces 17 are raised to such an extent that they have come into abutment against the underside of the vertically acting spring devices 15. Due to vertical oscillatory movement of the transporter, which in turn is due to the wave action, the horizontal sliding surfaces 17 move vertically up and down, colliding against the vertically acting spring devices 15. These shocks are absorbed by the vertically acting spring devices, without causing damage.

Fig. 13 illustrates the position during further deballasting of the transporter.

On account of the vertical forces from the lifting beams the vertically acting spring devices are compressed here. The transporter's vertical oscillatory movement here does not give rise to impacts, but instead is absorbed as variations in the compression of the vertically acting spring devices. As deballasting of the transporter increases the weight of the topsides is gradually transferred from the substructure to the transporter, and the forces between the friction clamps'lifting portions 14 and the lifting beams 31,31' gradually become so great that the transporter remains almost at rest in the vertical direction. The problem of impacts between the transporter and the topsides when the transporter is in the process of being brought into abutment against the topsides is thereby solved.

Fig. 14 illustrates a pair of lifting beams 31, 31'which together encircle the leg 4 with the friction clamp 14, forming a uniform lifting beam for lifting the friction clamp. Here too the vertically acting spring devices are compressed and fig. 14 thus shows the same relative position between the lifting beam, the friction clamp and the leg as in fig. 13.

When horizontal forces are applied the vertically acting spring devices will assume an oblique position. In the event of horizontal forces beyond a given value, depending on the actual design of the vertically acting spring device, this oblique position will increase with increasing vertical loading, with the result that the vertical loading gives rise to a negative spring effect in the horizontal direction. The horizontal sliding surfaces permit movement of the lifting beams, i. e. the transporter, in the horizontal direction, ensuring that

the horizontal forces which act on the vertical spring devices are kept so low that no negative spring effect occurs in the horizontal direction.

The transporter's horizontal movement can be absorbed in several ways. In connection with the description of fig. 2 it was mentioned that the oscillatory movement can be absorbed by means of damping devices which absorb the oscillatory movements with friction, but that in order to prevent the topsides from working loose from the transporter it is necessary to establish a rigidity in the actual securing means.

Horizontal damping movement can be achieved by providing a horizontally acting oscillation damper between the topsides 3 and the transporter 1. This is not shown in the figures, but an oscillation damper of this kind may comprise double-acting hydraulic cylinders where the cylinders'two sides communicate with each other via a choke valve.

Horizontal damping movement can also be achieved by giving the horizontal sliding surface 17 a certain amount of preselected friction. This can be done as simply as having two steel surfaces slide against each other, but may also be done by applying to the surfaces a coating intended for this purpose. A part of the oscillatory movement can thereby be absorbed by frictional forces. However, it is necessary to restrict these frictional forces in order to avoid inducing excessive horizontal forces on the vertically acting spring devices, as mentioned above.

A horizontal rigidity can be obtained by arranging a horizontally acting spring device between the topsides 3 and the transporter 1. This is achieved by means of the said horizontally acting spring devices 16 arranged in the vertical recesses 34 in the lifting beams 31, 31'. When the lifting beams are caused in pairs to encircle the legs 4 with the friction clamps 7, the horizontally acting spring devices are brought into abutment against the friction clamps. Since the friction clamps are securely connected to the legs 4, in reality the horizontally acting spring devices are brought into abutment against the legs, thereby damping the transporter's horizontal movement.

As mentioned with reference to fig. 2 the transporter may also be moored to the seabed, or also to the topsides or the substructure. In an actual case a combination of several of these functions may be employed in order to dampen the transporter's oscillatory movement.

In the illustrated embodiment the horizontally acting spring devices are securely connected to the lifting beams in their vertical recesses, and abut loosely against the legs with the friction clamps. From the above, however, it should be evident that in order to provide the horizontal rigidity between the transporter and the legs or the topsides, it is unimportant where the horizontally acting spring devices are attached, and that they therefore might just as well have been attached to the friction clamps and abutted loosely against the lifting beams in their vertical recesses.

Furthermore, it should also be evident from the above that in order to provide the vertical rigidity between the transporter and the topsides it is unimportant whether the vertically acting spring devices are attached to the friction clamps'lifting portions or the lifting beams. The latter solution is equally good, and in this case the sliding surfaces would have to be located between the vertical spring devices and the friction clamps'lifting portions. The important factor is that the vertically acting spring device 15 and the horizontal sliding surface 17 are arranged in series between the topsides 3 and the transporter 1.

Both the vertically acting spring devices and the horizontally acting spring devices preferably have a progressive characteristic. An advantageous embodiment is obtained by constructing the spring devices from alternately arranged lamellae of a rigid material, in practice steel, and an elastomer, usually rubber. With reference to figs. 9 and 10, the steel lamellae are indicated by reference numerals 18 and 20 for the vertically acting and horizontally acting spring devices respectively, while the corresponding elastomer lamellae are indicated by reference numerals 19 and 21. The vertically acting spring devices 15 are designed as rings which are located along the circumference of the friction clamps'lifting portions 14, and are composed of lamellae with the surfaces parallel to the ring's radial direction.

The horizontally acting spring devices 16 are designed as rings which encircle the legs 4, and are composed of lamellae which form concentric rings around the legs 4. The horizontally acting spring devices thereby provide a radial spring effect for the leg 4 in all horizontal directions. Both the vertically and horizontally acting spring devices could, however, have been designed in other ways, for example as groups of separate blocks composed of lamellae of steel and an elastomer.

The springing properties are taken care of by the elastomer lamellae, while the steel lamellae counteract lateral protuberance of the elastomer lamellae.

By adapting the spring devices'external geometrical measurements and the lamellae's thickness and number it is possible to obtain the desired spring characteristic.

By means of correct sizing of the vertically and horizontally acting spring devices, and taking into account the other above-mentioned factors which also influence the transporter's oscillatory movement, the object is achieved that the attachment of the transporter to the topsides is carried out with a rigidity in the area which with reference to fig. 2 is indicated by C, and the problem of oscillations of the transporter after attachment to the topsides due to excitation from the waves can thereby be solved.

When employing the friction clamps 7, before the transporter is moved in under the topsides, these are arranged around and clamped into abutment against each of the legs 4 both on the top 10 and the underside 11 of the cuts 9. The friction clamps'7 total frictional force above the cuts 9 must be arranged to be greater than the weight of the topsides 3, thus ensuring that the friction clamps are capable of holding the topsides.

The friction clamps'total frictional force below the cuts is arranged to be greater than or equal to the buoyancy required by the transporter 1 in order to lift the topsides 3 to a height where the risk of collision between opposing cut surfaces in the cut 9 due to the vertical movement obtained by the transporter 1 and the topsides 3 after the topsides is released from the substructure and attached to the transporter is not present. In other words: after the topsides has been released from the substructure and lifted up by the transporter the transporter and the topsides will obtain a vertical oscillatory movement which is hydrodynamically dampened by the surrounding water.

The frictional force between the friction clamps and the legs below the cuts must be so great that the topsides does not work loose before the transporter's buoyancy is so great that the transporter and the topsides will not move downwards to such an extent that the cut surfaces on the top of the cuts collide with the cut surfaces on the underside of the cuts. This frictional force can be calculated by an analysis of the transporter's buoyancy and dynamic behaviour in the water.

Analyses show that the friction clamps'required frictional force below the cuts will always be less than the weight of the topsides, and the friction clamps are therefore placed with most of the friction portions above the cuts.

After the transporter has been deballasted to such an extent that it is almost at rest against the friction clamps'lifting portions, as mentioned with reference to figs. 13 and 14, the transporter is further deballasted to a buoyancy which exceeds the weight of the topsides. When the transporter's buoyancy is equal to the sum of the weight of the topsides 3 and the frictional force between the friction clamps 7 and the legs 4 on the underside 11 of the cuts 9, the friction clamps on the underside of the cuts are released, thereby causing the topsides to be lifted up from the substructure, which is given by an equation of equilibrium. By providing the frictional force below the cuts in advance as described above, the cut surfaces on the top of the cuts are prevented from colliding with the cut surfaces on the underside of the cuts after the topsides has been released from the substructure. The problem of collisions between the topsides and the substructure when the topsides is in the process of being released from the substructure is thereby solved.

Since there will always be a certain amount of uncertainty attached to the extent of frictional force actually present between the friction clamps and the legs, it is preferred that the frictional forces should be provided in rather larger amounts than necessary both above and below the cuts, and that the frictional force between portions of the legs 4 below the cuts 9 and corresponding portions of the friction portions 8 should be reduced after the transporter has been given the buoyancy required for the friction portions to be released.

Fig. 15 illustrates the lifting beam 31 and the friction clamp with the lifting portion 14 and the friction portion 8 after the platform leg is released from the friction clamp on the underside 11 of the cut. The cut surface on the top 10 of the cut is indicated here by reference numeral 12, while the cut surface on the underside 11 of the cut is indicated by reference numeral 13.

In the above the invention has been explained with reference to a specific embodiment with individual variants. It is obvious, however, that other embodiments are possible, associated, for example, with the design of the transporter, where it is possible, for example, to employ two barges with intermediate cross beams instead of the described transporter.