GB2414716A

GB2414716A - Method for moving a structure which is at least partially underwater upwards

Info

Publication number: GB2414716A
Application number: GB0511199A
Authority: GB
Inventors: Waard Onno Ludolf De
Original assignee: Heerema Marine Contractors Nederland BV
Current assignee: Heerema Marine Contractors Nederland BV
Priority date: 2004-06-01
Filing date: 2005-06-01
Publication date: 2005-12-07
Anticipated expiration: 2025-06-01
Also published as: NL1026301C2; NO20052638D0; NO20052638L; GB0511199D0; GB2414716B

Abstract

The disclosure relates to a method and device (20) for moving a structure (2) which is at least partially underwater, in particular an offshore platform, upwards. In the method, first of all at least two float bodies (22a, 22b) are connected to the structure by means of associated arms. (24a,24b,fig 2). Then, the float bodies are moved into a position which is suitable for the upwards movement. Next, the buoyancy of the float bodies is increased in order to float the structure.

Description

Method for moving a structure which is at least partially underwater

upwards The present invention relates to a method for moving a structure which is at least partially underwater upwards. The invention also relates to a float assembly for moving a structure which is at least partially underwater upwards. Furthermore, the invention relates to a structure connected to a float assembly.

Methods and devices for moving structures which are positioned in the water upwards are known.

In practice, structures at sea, such as oil or gas production platforms (offshore platforms) have an economic or technical lifespan, after which the structures often have to be moved away and/or broken up. The structure often rests on the (sea) bed, and in order to be moved away it must often first be moved upwards.

According to a known method for moving an offshore platform upwards, at least one float tank is coupled to the offshore platform underwater. The buoyancy of the float tank is then increased, and the offshore platform is removed from the bed on which it is standing. Then, the buoyancy of the assembly comprising the float tank and the offshore platform is increased further, so that the offshore platform is moved upwards.

One significant drawback of the known method is that the float assembly also has to be coupled to the structure below the surface of the water. This generally means that manually a coupling has to be made between the float tank and the offshore platform underwater. This can be done, for example, by underwater welding or by hydraulic clamps. However, working underwater is technically complex. Moreover, it is necessary to take into account weather conditions, such as wind and current.

Furthermore, the pressure increases with depth, making it more difficult to work at greater depths. If divers need to work underwater, there may also be safety risks for the divers. The risks increase as the depth becomes greater. Working underwater is also expensive, partly for the reasons described above.

Furthermore, the vertical buoyancy required to lift the platform off the ground is considerable at the location of the points where the float assembly is secured to the drilling platform, and it is quite possible that the platform will need to be reinforced at the location of the connection points. In any event, the offshore platform has to be the subject of extensive structural inspection at the connection points.

Another significant drawback of the known method is that the float tank, after it has been secured to the offshore platform, intersects the waterline in order to lift the structure off the bed and then tow it. In the event of a storm at sea, this float tank is subject to very high levels of load from the waves, and consequently the known method is only suitable for use when there are no waves or the waves are low. However, the towing operation may last many days, during which period the weather can deteriorate, which can give rise to high levels of wave loads on the float assembly.

A further drawback of the known method is the height by which the offshore platform is moved upwards is limited. This height is less than the height of the float tanks.

Consequently, there is a demand for a method for moving structures which are least partially underwater upwards, in which the amount of work which has to be performed below the water-line is relatively small. Further, this work preferably should not involve any complicated actions. Furthermore, there is a demand to obviate the exertion of high (floating) forces on the platform underwater. The invention aims to satisfy at least one of these demands.

For this purpose, the invention firstly provides a method for moving a structure which is at least partially underwater, in particular an offshore platform, upwards, the method comprising the steps: (a) coupling a float assembly to the structure, the float - 3 - assembly comprising at least two float bodies, which are provided with a variable buoyancy and are coupled to the structure at respective connection points by means of associated arms; (b) bringing the float assembly into a position which is suitable for moving the structure upwards; and (c) increasing the buoyancy of the float bodies in order to move the structure upwards.

One advantage of the present invention is that the float assembly can be coupled to the structure above the water-line, and that there is no need to effect connections between the float assembly and the structure below the surface of the water.

In this context, it is preferable for the float assembly, in the position required for the upwards movement, to be in a fixed position with respect to the structure, so that the upwards force of the float body can be transmitted to the structure without the float body moving with respect to the structure. The upwards force of the float body is transmitted to the structure at the connection points.

Another advantage of the method according to the invention is that a structure which is at least partially underwater can be moved upwards easily and inexpensively.

A very wide range of structures can be moved upwards using the method according to the invention, but the structure is generally an offshore platform. The structure can rest on the bed of an ocean, a sea or a lake in which the structure is located. Therefore, the structure can also be a floating structure which needs to be floated. The structure may be an oil or gas production platform or drilling platform, a wind turbine, a mooring device or a storage tank or any other structure which is at least partially underwater.

In step (a), the float assembly is coupled to the structure. In this context, it is possible for the arms to be connected to the structure by means of respective connection couplings. It is also possible for the connection couplings to be designed in such a manner that they can be coupled directly to the structure, or that coupling pieces, to which connection couplings are connected, are connected to the structure prior to step (a). This can take place, for example, during construction of the structure, or immediately prior to step (a).

The float assembly comprises at least two float bodies, with a respective arm secured to each float body. The float bodies will generally be designed as a reservoir with a fixed shape.

However, it is also possible for the float bodies to be inflatable. The float bodies have a buoyancy which can be varied, for example, by means of a pump which can pump air and/or water into and out of the float body. To increase the buoyancy, compressed air is pumped into the float body. To reduce the buoyancy, water is pumped and/or admitted into the float body. In this context, it is possible to provide a pump which is arranged directly on the float body or is positioned above the water, as will be immediately clear to a person skilled in the art. In the latter case, an air feed line will extend from the pump to the float body. It is also possible for the float assembly to comprise a compressed-air tank for filling the float body with air. Openings with controllable valves can be provided in the float bodies for admitting water. Float bodies of this type are known in the specialist field.

It is possible for the float bodies to float on the surface in step (a). It is also possible for the float bodies to be partially underwater or completely underwater. During step (a), the float bodies will generally be manoeuvred towards the structure during favourable weather conditions, in particular with regard to wind, waves and current.

The arms of the float assembly may be straight or curved. The arms may be designed as a lattice girder or as a profiled section or in any other suitable form. The arms may be made from steel or another structural material. The arm and the float body can be fixedly secured to one another. However, it is also possible for the arm and the float body to be released from and attached to one another. In that case, the arms can be connected - 5 - to the structure first of all, after which the float bodies are connected to the arms. There may be one connection coupling per arm for coupling the arm to the structure, but it is also possible for an arm to have a plurality of connection couplings.

These connection couplings can then be coupled to the structure at the same height, with the connection couplings positioned at a predetermined horizontal distance from one another.

In step (b), the float assembly is positioned in a suitable position with respect to the structure. In this context, it is preferable for the float bodies to be moved downwards with respect to the structure.

During step (c), as the buoyancy is increased, the structure will not in the first instance generally be moved upwards.

Often, the total upward force acting on the structure will first have to exceed the downwards force. For this purpose, the upward force of the float bodies has to exceed a defined limit; the limit is dependent, inter alla, on the weight of the structure.

According to a preferred embodiment of the method according to the invention, in step (b) the float bodies are brought to a predetermined depth below the surface of the water.

If the float bodies are floating at the surface in step (a), they will first be moved to a defined depth before moving the structure upwards. This can be done by adding ballast to the float bodies. The structure can then be moved upwards over a distance which is substantially equal to the depth to which the float bodies have been brought in step (b). It follows from this that the deeper the float bodies are positioned, the further it will be possible for the structure to be moved upwards.

In a further embodiment, the float assembly is positioned in such a manner with respect to the structure that the structure is held in balance during step (c).

In this case, the orientation of the structure with respect to its surroundings will remain substantially identical. In many - 6 - cases, it will be desirable for the structure to be held upright while it is being moved upwards. For this purpose, it may be necessary for the float bodies to be coupled to the structure at predetermined positions. It may also be expedient for the buoyancies of the float bodies to be varied independently of one another in a controlled way during step (c), so that the structure is kept permanently upright. However, it is also possible for the orientation of the structure to be varied during step (c) , in order for the structure to be rotated about a substantially horizontal axis. In this context, consideration may be given to transporting the structure in an orientation which is different from its initial orientation in the water.

In another preferred embodiment, in step (a) two float bodies are coupled to the structure on opposite sides of the structure.

If the structure is symmetrical, it will be easy to reach an equilibrium situation by coupling the float bodies to the structure on opposite sides.

The arms are preferably connected to the structure by means of respective connection couplings.

This has the advantage that the arms can be connected to the structure in a technically simple and reliable way.

Preferably, in step (b) the at least two float bodies are connected to one another by means of a connecting member for holding the float bodies in a fixed position with respect to one another.

This fixes the positions of the float bodies with respect to the structure in a simple way. Furthermore, the positions of the float bodies with respect to one another are positioned in a simple way. Furthermore, this has the advantage that there is no need to make a connection between the float assembly and the structure underwater.

It is possible for the connecting member to be designed as a rocker bar. It is also possible for the connecting member to be designed as a cable, a chain, a wire or a cord, or any other suitable connecting member. It is possible for the connecting member to comprise a plurality of parts.

With regard to the distribution of forces, it is advantageous for the connecting member to be arranged at a substantial distance from the connection points, preferably in the vicinity of the float bodies. During operation, the connecting member will absorb substantially horizontal forces. If the distance between the connection points and the location where the connecting member is connected to the float body is at a maximum, this has the advantage that the forces which occur in the connecting member during operation are minimal. It is in this case possible to apply a prestressing force in the connecting member. This has the advantage that the float bodies are pulled taut onto the structure and will be very securely fixed with respect to the structure.

Preferably, each arm can pivot about a substantially horizontal axis extending through the respective connection point, and in which in step (b) the arms are rotated through a predetermined angle about the respective axes, with the float bodies being moved downwards with respect to the structure.

This embodiment has the advantage that the respective arms can be of simple design. The float bodies can easily be brought to the desired depth by rotating the arms with respect to the structure about the respective axes which extend through the connection couplings.

If an arm is coupled to the structure at more than one connection point, the axis of rotation will extend through all the connection couplings.

A person skilled in the art will readily understand that it is also possible for the arms to be made not rotatable but extendable. In this case, in step (a) the arms are connected to the structure in a substantially vertical orientation. If the - 8 - arms are extended in step (b), the float bodies will be moved downwards with respect to the structure. Another possible option is for the arms to be designed in split parts which are connected to one another by means of hinges, in which case the split parts can be folded open and, after the split parts have been folded open, the hinges are fixed in a predetermined position. The float bodies can be brought to the desired depth in this way too, while during the upwards movement they can transfer their upwards force to the structure at the respective connection points. A person skilled in the art will readily understand that there are also other ways in which the float body can be brought to a desired depth.

In step (b), the arms are preferably rotated with respect to the structure in such a manner that the float bodies substantially bear against the structure.

The float bodies, when they come to bear against the structure, will be unable to rotate further and will be held in place by the structure. In this way, each float body is fixed with respect to the structure in one direction of rotation of the arm. In this position the arms can only still rotate away from the structure, with the result that the float bodies move away from the structure. If the float bodies are then connected to one another by means of the connecting member, this movement is also prevented. The float bodies will therefore be completely fixed with respect to the structure.

Prior to step (b), it is preferable for force-absorbing means to be arranged between the float assembly and the structure.

The advantage of this is that the forces which the float assembly and the structure can exert on one another, for example under the influence of wave loads, can be transmitted without damage to the structure or the float assembly. A further advantage is that it is easier to set the force which the float assembly and the structure exert on one another to a predetermined value. The force-absorbing means may be fenders.

The force-absorbing means may be designed as pieces of wood or - 9 - as pieces of another material which has a certain resilience.

Preferably, during step (c) the float assembly is held in a predetermined horizontal position with respect to the structure by at least one guide. This makes it easy to prevent movements in the horizontal direction between the drive assembly and the structure.

In a further preferred embodiment, the connection points are located above the surface of the water.

In practice, this is the location where it is easiest for the arms to be coupled to the structure. There is good access to the connection points for producing the coupling between the arms and the structure above the surface of the water. The drawbacks which, as described above, are associated with the known method of working underwater and coupling the float assembly to the structure underwater are eliminated when working above water.

The float bodies preferably have a positive buoyancy during step (a).

Since the float bodies then contain little or no water, they are relatively lightweight and the float bodies are relatively easy to manoeuvre with respect to the structure.

A person skilled in the art will readily understand that the words "positive buoyancy" in this context mean that the float bodies can float and, if positioned underwater, seek to move towards the surface of the water.

In a further preferred embodiment, in step (b) the buoyancy of the float bodies is reduced in order for the float bodies to be moved downwards with respect to the structure.

As a result of the float bodies becoming heavier, the float bodies will acquire a negative buoyancy and will move downwards under the influence of this negative buoyancy. As a result, the float bodies are sunk without much effort. Since they are - 10 already coupled to the structure, they will come to a standstill at a predetermined depth below the surface of the water. The float bodies can then be connected to one another.

The method according to the invention preferably comprises a step (d) in which the structure with the float assembly connected to it is transported from a first location to a second location, after which the buoyancy of the float bodies is reduced, in order to lower the structure.

The floating structure with the float assembly coupled to it is in that case transported in its entirety. During transport, the structure can be towed or pushed by a tug boat or other vessel.

The float bodies will preferably remain below the water-line, in order to keep the loading from waves at a low level. If there are not many waves, the structure can also be towed with the float bodies closer to or at the water-line. The towing speed will in this case increase, and furthermore, it will be possible to tow the structure even through relatively shallow water. If high waves are forecast, it may be desirable for the float bodies to be moved back deeper below the surface of the water.

It is possible for the structure to be sunk at a location where the depth of the water is so great that the structure can remain there permanently without causing problems. In this case, it is possible for the structure to be sunk together with the float assembly.

The second location preferably has a shallower depth than the first location, in which case the float assembly is uncoupled from the structure after the buoyancy has been reduced.

In this way, the structure drops back onto the bed, with a larger part of the structure above the surface of the water than at the first location. Consequently, it is easier to carry out work on this part of the structure which is located above the water. Moreover, a second cycle of moving the structure upwards can be commenced at the second location. In this case, the cycle comprises the steps of: coupling the arms to the structure, bringing the float bodies into a desired position, moving the structure upwards, transporting the structure to a shallower location, lowering the structure again until it stands on the bed and breaking up a part of the structure which projects above the surface of the water. At the start of the second cycle, the float assembly is coupled to the structure, with the connection points being located at a lower level on the structure than the connection points used in the first cycle. Consequently, in the second cycle the structure can be floated to a higher level than in the first cycle. These cycles can be repeated until the structure is at a desired level above the surface of the water.

After step (c) has ended, it is preferable for the structure to be at least partially broken up.

The breaking up can be carried out, for example, by dismantling the structure, demolishing it or taking it apart. The top part of the structure is located above the surface of the water and is therefore readily accessible to workers and equipment.

Breaking up part of the structure makes the structure more lightweight, which means that it can easily moved further upwards by the float assembly in a second cycle. If a plurality of upward movement cycles are used, the top part of the structure can be broken up during each cycle.

In a further preferred embodiment, in step (b) two float bodies are connected to one another by means of at least two connecting members, with a closed loop of float bodies and connecting members arranged between them being formed around the structure.

This has the advantage that the upward forces of the float bodies acting on the structure can be distributed uniformly over the periphery of the structure.

A person skilled in the art will readily understand that the loop is located substantially in one horizontal plane. However, there may be relatively minor differences in height between the float bodies. In some cases, it is possible to use a large number of float bodies and connecting members, for example in the case of relatively large structures. - 12

It has been found that the invention is eminently suitable for moving offshore platforms upwards.

The invention also relates to a float assembly for moving a structure which is at least partially underwater upwards, the float assembly comprising: - at least two float bodies provided with a variable buoyancy; - at least two arms, which are each connected to an associated float body, it being possible for the float bodies to be coupled to the structure at respective connection points by means of the arms; and - a connecting member for holding the float bodies in a fixed position with respect to one another.

This float assembly can be used to move a structure which is at least partially underwater upwards in a particularly simple way.

Moreover, the float assembly is suitable for carrying out the method according to the invention.

The float bodies are preferably elongate in form, in particular cylindrical in form or in the shape of a box.

Preferably, the float bodies have respective main longitudinal axes which extend substantially parallel to the substantially horizontal axes extending through the respective connection points. In practice, this gives a low towing resistance.

Preferably, the float assembly is provided with a first connecting member coupling, and the second float body is provided with a second connecting member coupling for coupling the connecting member to the first float body and the second float body.

Preferably, the first and second connecting member couplings are designed to allow a remote-controlled connection to be produced between the first and second float bodies. - 13

If the coupling can be controlled from a location above the surface of the water, this has the advantage that there is no need for a diver or robot to be present when making the connection between the connecting member and the float bodies.

This makes operation easier. A person skilled in the art will readily understand that there are numerous conceivable embodiments of a connecting member coupling of this type.

Preferably, each arm can be coupled to the structure at at least two connection points on the structure. In this way, in the case of a structure with four legs, a float body can be connected to two legs. In this case, the width of the arm may be at least equal to the distance between the two legs. It is in this way possible to produce a coupling which is torsionally rigid about an axis of rotation extending through the longitudinal axis of the arm.

The invention also relates to an arm, described as a component of the float assembly.

The invention also relates to a structure connected to a float assembly.

Further preferred embodiments of the device are described in the claims. The invention is explained in more detail below with reference to the appended, non-limiting drawing, in which: Figure 1 shows a diagrammatic side view of the structure with a float assembly coupled to it; Figure 2 shows a diagrammatic side view of the structure with float bodies moved downwards; Figure 3 shows a diagrammatic side view of the structure with float bodies which are connected to one another by means of a connecting member; Figure 4A shows a diagrammatic side view of the floating structure with float assembly, with the float bodies underwater; Figure 4B shows a diagrammatic side view of the structure with float assembly moved upwards, with the float bodies located partially above the water; Figure 5 shows a diagrammatic side view of the sunken - 14 - structure with the float assembly; Figure 6 shows a diagrammatic side view of the partially broken-up structure; Figure 7 shows a diagrammatic side view of the partially broken-up structure with the float assembly coupled to it; Figure 8 shows a diagrammatic side view of the floating, partially broken-up structure with the float assembly coupled to it; Figure 9 shows a diagrammatic side view of the floating structure with the float assembly, towed by a boat; Figure 10 shows a diagrammatic side view of the floating structure with the float assembly, positioned on a semisubmersible vessel; Figure 11 shows a perspective view of an alternative embodiment of the float assembly; Figs. 12a and 12b show a detailed view of an embodiment of a connection coupling; Figure 13 shows a detailed view of an embodiment of the float bodies with the connecting member; Figure 14 shows an embodiment of the float assembly positioned as a continuous loop around the structure; Figure 15 shows an embodiment of the float assembly, provided with stabilizer members; Figure 16 shows an embodiment of the floated float assembly provided with stabilizer members; Figure 17 shows a cross section, seen from above, of an alternative embodiment of the float assembly; and Figure 18 shows a detailed view in the direction indicated by line A-A in Figure 17; Figure 19 shows a detailed view in the direction indicated by line B-B in Figure 17; and Figure 20 shows a diagrammatic side view of an arm; and Figures 21a, 21b and 21c show diagrammatic side views of a coupling device between an arm and the structure.

Identical reference numerals denote identical components or components with an identical or similar function. Arrows without a reference numeral indicate directions of movement of components. -

Figure 1 shows a diagrammatic side view of a float assembly according to the invention which, in the embodiment shown, is coupled to an offshore platform 2. The structure 2 is at a first location 80 in water 6, resting on a bed 8, with a top section 12 above the water-line 10 and a bottom section 14 below the water-line 10. The float assembly 20 comprises two float bodies 22a and 22b. The float assembly 20 may also comprise more than two float bodies 22a, 22b. The float bodies 22a and 22b are coupled to the structure 2 at connection points 28a, 28b. The float bodies 22a, 22b are coupled to the structure 2 by means of two respective arms 24a and 24b.

Respective first ends 26a, 26b of the arms 24a, 24b are coupled to the structure 2 at respective connection points 28a, 28b. In the embodiment shown, the connection points 28a, 28b are above the water-line 10. The arms 24a, 24b are hingedly coupled to the structure 2. Respective hinged couplings 38a, 38b between the arms 24a, 24b and the structure 2 arearranged at the connection points 28a, 28b, so that the arms 24a, 24b can pivot about the connection points 28a, 28b.

Respective second ends 30a, 30b of the arms 24a, 24b are connected to the float bodies 22a, 22b.

The structure 2 has legs 32a, 32b. Two legs 32a, 32b are illustrated in this figure. A person skilled in the art will recognize that in practice the structure may also comprise one leg. This leg may then have a considerable cross section, in order to increase the stability of the structure 2. In this case, the leg may, for example, be a hollow tube. It is also possible for the structure to have three or more legs. Any number of legs is possible, provided that the structure 2 can rest on the bed with sufficient strength and stability. The structure 2 may be produced from concrete or steel or any other suitable construction material. The structure 2 may be a framework structure, as illustrated here, or any other type of structure which is partially underwater. - 16

The float bodies 22a, 22b have a variable buoyancy. The buoyancy is varied by varying the quantity of water in the float bodies 22a, 22b. When a float body 22a, 22b is completely full of water, the buoyancy will be at a minimum and the float body 22a, 22b will sink or at least move downwards underwater. When the float body 22a, 22b is completely full of air, the float body 22a, 22b will be in a floating position or, if the float body 22a, 22b is underwater, will seek to float upwards. The buoyancy is varied by means of a pump device 34. The pump device 34 is located on the structure 2 and is connected to the respective float bodies 22a, 22b by means of collection lines 36a, 36b. The collection lines 36a, 36b comprise lines for air (not shown) to pass through and lines for transmitting control signals from the pump device 34 to the float bodies 22a, 22b. The pump device 34 can pump compressed air to the float bodies 22a, 22b in a controlled way. Water valves (not shown) in the float bodies 22a, 22b can be controlled with the aid of the control signals.

During operation, if the buoyancy of the float bodies needs to be increased, the pump device 34 will pump air into the float bodies 22a, 22b. At the same time, the water valves in the float bodies 22a, 22b are opened. If the buoyancy of the float bodies 22a, 22b needs to be reduced, the water valves in the float bodies 22a, 22b are opened, and at the same time air valves (not shown) in the float bodies 22a, 22b are opened in order to allow the air to escape from the float bodies 22a, 22b. A configuration of this type is known from the prior art. It is also possible to use a container holding compressed air to feed air to the float bodies 22a, 22b. A person skilled in the art will recognize that other configurations are also possible for varying the buoyancy of the float bodies 22a, 22b.

In this embodiment, the shape of float bodies 22a, 22b is fixed.

In this case, the float bodies 22a, 22b are substantially cylindrical, having a defined diameter and length. It is also possible for the float bodies 22a, 22b to be designed as air bags (not shown). In this case, the air bags may then be hingedly connected to the ends 30a, 30b of the arms 24a, 24b. - 17

The method according to the invention will be explained in more detail in Figs. 2-10.

Figure 2 shows the float assembly 20 with the float bodies 22a, 22b moved downwards with respect to the structure 2. The float bodies 22a, 22b have rotated about the respective connection points 28a, 28b in the direction indicated by the respective arrows 40a, 40b. In this position, the float bodies 22a, 22b are underwater and are completely filled with water. The arms 24a, 24b extend substantially parallel to the respective legs 32a, 32b of the structure 2, or extend at a small angle with respect to the legs 32a, 32b. In this position, the float bodies 22a, 22b are touching and bearing against the respective legs 32a, 32b of the structure 2.

Figure 3 shows the float assembly 20, with a connecting member 42 arranged between the first float body 22a and the second float body 22b. The connecting member 42 is connected to the projections 50a, 50b of the float bodies 22a, 22b. The connecting member 42 may be a profiled bar, a lattice girder, but may also be a weak connecting member, such as a cord, a cable, a chain or a wire. The connecting member 42 may be made from any suitable material. The float bodies 22a, 22b are positioned against the respective legs 32a, 32b.

One aim of the connecting member 42 is to hold the float bodies 22a, 22b against the structure 2. The float bodies 22a, 22b can be directly connected to the legs 32a and 32b. However, these connections are not used to absorb the buoyancy force of the float bodies 22a, 22b, but rather are merely intended to hold the float tanks against the structure 2. The connecting member 42 therefore holds the float bodies 22a, 22b in a fixed position with respect to the structure 2. In this position, the float bodies 22a, 22b can no longer rotate about the connection points 28a, 28b.

It is also possible for the connecting member 42 to be connected to the respective arms 24a, 24b rather than to the respective float bodies 22a, 22b. In this case, a horizontal force on the - 18 connecting member 42 will increase as the distance between the connecting member 42 and the connection points 28a, 28b becomes smaller. The float assembly 20 comprises at least two float units 25a, 25b. Each float unit comprises a float body 22a, 22b and a respective arm 24a, 24b. The connecting member 42 is connected to each float unit 25a, 25b in order to fix the position of the float units 25a, 25b.

Figure 4a shows the structure 2 in a position in which it has been moved upwards. The float bodies 22a and 22b have been completely or partly filled with air and are located closer to the surface 10 of the water than in the position shown in Figure 3. The float bodies 22a, 22b are underwater. This is the preferred position for transportation at sea. This floating structure is stable if the centre of gravity is below the centre of buoyancy.

Figure 4b shows the structure 2 in a position after further upwards movement, with the float bodies 22a, 22b partly above the water-line 10.

In both Figure 4a and Figure 4b, the structure 2 is has moved upwards with respect to the initial position shown in Figure 1, in which the structure 2 was resting on the bed 8. In this position shown in Figure 4b, the unit made up of structure 2 and float assembly 20 is in a floating state. The upwards forces of the float bodies 22a, 22b are exerted on the structure 2 at the location of the connection points 28a, 28b. In this state, the structure is ready to be transported, for example to a second location 82 where the depth of the water is shallower.

Figure 5 shows the unit comprising structure 2 and float assembly 20 at a second location 82, where the depth of the water is shallower than the depth of the water shown in Figs. 1-4. The second location may, for example, be inshore or at a location near the coast, protected from the beating of the waves, such as for example a fjord in Norway, or behind an island, etc. The float bodies 22, 22b have first of all been filled with water in such a manner that the structure 2 no - 19 - longer floats, but rather has sunk to the bed 8. Then, the connecting member 42 is removed, after which the water is removed from the float bodies, so that they float upwards and the arms 24a, 24b are in a slightly spread position with respect to the structure 2. Since the depth of the water is less, a larger part of the structure 2 projects above the water than in the initial position shown in Figure 1.

Figure 6 shows the structure 2 which has been partially broken up. The float assembly 20 has been detached from the structure 2. If the structure 2 is dismantleable, it can be dismantled. If the structure 2 is not dismantleable, the structure 2 can be demolished by techniques which are customarily used in the specialist field. The top section 12 may, for example, be sawn or cut off the bottom section 14 as a single piece. However, it is important that if a second upwards movement cycle is still to take place, there still be sufficient positions for connection points for securing the structure 2 thereto during the second cycle.

Figure 7 shows the partially broken-up structure 2 at the start of a second cycle. The float bodies 22a, 22b are coupled to the structure 2 again, at the location of respective connection points 28a, 28b, which in the second cycle are closer to feet 44a, 44b of the structure 2 than the connection points 28a, 28b from the first cycle. Then, the arms 24, 24b will again be rotated towards the structure 2, so that the float bodies 22a, 22b once again come to bear against the structure 2. Then, a connecting member 42 will once again be fitted so as to fix the float bodies 22a, 22b with respect to the structure 2. A person skilled in the art will readily understand that in the case of an offshore platform which is wider at the bottom than at the top, in the second cycle the float bodies 22a, 22b need to be at a greater distance from one another than the float bodies 22a, 22b in the first cycle, which means that the connecting member 42 in the second cycle must be of a greater length in order to enable the float bodies 22a, 22b to be connected to one another.

This is not the case if the offshore platform has parallel vertical legs. - 20

The float bodies 22a, 22b are filled with water again in order to move downwards with respect to the structure. After they have been connected by the connecting member 42, the float bodies 22a, 22b will be pumped empty again or filled with compressed air, so that the buoyancy increases. When the buoyancy is sufficient to move the structure 2 upwards, the structure 2 will be lifted off the bed 8 and move upwards.

Figure 8 shows the partially broken-up structure 2 in a floating position. The structure 2 has moved further upwards than in the first cycle, with the result that the structure 2 has a lesser draft than during the first cycle. The floats 22a, 22b are at a lower position with respect to the structure 2 than during the first cycle. The structure 2 can then be transported again, to a third location, the depth of which will be shallower than that of the second location 82.

Figure 9 shows a diagrammatic side view of the structure 2 with the float assembly 20 during transportation, towed by a tug boat 46. Towing by boat is a safe, tried-and-tested form of transport.

Figure 10 shows a diagrammatic side view of the unit comprising the structure 2 with the float assembly during transport, positioned on a semi-submersible vessel 48. When the structure 2 has moved so far upwards that it has only a relatively low draft, the unit comprising the structure 2 and the float assembly 20 can be positioned over a semi- submersible vessel 48 which has moved downwards. Then, the buoyancy of the semisubmersible vessel 48 is increased again, so that the semi- submersible vessel 48 moves back upwards. The structure 2 comes to rest on the semi-submersible vessel 48. This vessel may be a dry dock or a semi-submersible ship. Other types of semi submersible vessels are also possible.

Figure 11 shows a perspective view of the float assembly 20, connected to the structure 2. The structure 2 is only diagrammatically depicted here, for the sake of clarity. The - 21 float bodies 22a, 22b are cylindrical and are of a predetermined length. The length is substantially greater than the diameter.

The cylindrical float bodies 22a, 22b extend substantially horizontally. Projections 50a, 50b, which lie in line with the respective centre axes 52a, 52b of the cylindrical float bodies 22a, 22b, are secured to the respective ends of the cylindrical float bodies 22a, 22b. The projections 50a, 50b form the connection points of the float bodies 22a, 22b and are discussed below with reference to Figure 13. The float bodies 22a, 22b may be secured rotatably to the arms 24a, 24b, but may also be fixed in a defined orientation with respect to the respective arms 24a, 24b.

The arms 24a and 24b are shown here as framework structures. A person skilled in the art will recognize that other embodiments of the arms 24a, 24b are also possible, such as arms formed as a single piece or in the form of profiled sections. The arm 24a is coupled to the structure 2 at two connection points, 28al and 28a2. Likewise, the arm 24b is coupled to the structure 2 at two connection points 28bl and 28b2. This produces a coupling between the float bodies 22a, 22b and the structure 2, preventing the float bodies 22a, 22b from rotating with respect to the structure 2 about respective axes 54a, 54b which extend through the arms 24a, 24b.

Figs. 12a and 12b show detailed views of an embodiment of a connection coupling 38 at a connection point 28. The connection coupling 38 is connected to a leg 32 of the structure 2. A clamp 62 is arranged around the leg 32. A hinge 64 is fixedly connected to the clamp 62. The arm 24 is pivotably connected to the limb 32 by means of the hinge 64. In practice, hinges of this type can be fitted around a plurality of legs 32. In the embodiment shown in Figure 11, two connection couplings 38 per arm will be used. A person skilled in the art will readily understand that various types of couplings 38 can be used, provided that the arm 24 can pivot about the leg 32 and provided that the upward forces of the float bodies 22a, 22b can be transmitted to the structure 2 via the couplings 38. - 22

One advantage of using a connection coupling 38 is that such a coupling can easily be connected to the structure 2 and can easily be detached from it again. The rigidity of the structure 2 itself is used during the upwards movement of the structure by virtue of the fact that the connection coupling 38 is coupled to the leg 32.

Figure 13 shows a diagrammatic view of the float bodies 22a, 22b with two connecting members 42a, 42b. The connecting members 42a, 42 comprise respective arms 66a, 66b, which are hingedly connected to the projections 50bl, 50b2 of the float body 22b at respective first ends 70a, 70b. In the vicinity of the respective ends 72a, 72b there are cutouts 68a, 68b which can form a coupling to the projections seal, 50a2 of the float body 22a. A person skilled in the art will quickly understand that there are numerous other embodiments of a coupling of this type.

Figure 14 shows another embodiment of the float assembly. In this case, the structure 2 has a large number of sides 74a, 74b, 74c, 74d, etc. A number of float bodies 22a, 22b, 22c, 22d, etc. are arranged around the structure 2. The float bodies 22a, 22b, 22c, 22d are connected to connecting members 42a, 42b, 42c, 42d, etc. in order to form a continuous loop around the structure 2.

In this way, even relatively large structures can be moved upwards.

Figs. 15 and 16 show an embodiment according to the invention in which stabilizer members 76a, 76b are used to increase the stability of the structure 2 that is to be moved upwards. The stabilizer members 76a, 76b are connected to the respective float bodies 22a, 22b by means of stabilizing connections 78a, 78b.

During the upwards movement, should the structure 2 together with the float assembly 20 coupled to it lean over to the left, the stabilizer member 76a will be pulled downwards via the stabilizing connection 78a. Then, the stabilizing member 76a will develop an upward reaction force on the float body 22a, which will pull the structure 2 upright again. In the event of - 23 - leaning to the right, a similar reaction will occur via the stabilizer member 76b. One condition for correct operation is for the stabilizing connections 78a, 78b to be able to transmit the forces, with the result that their length will have to be continuously reduced during the upwards movement.

Figure 17 shows an alternative embodiment of the float assembly and the structure 2, in which the structure 2 has eight legs 32a to 32h. Force-absorbing means, such as fenders 86al, 86a2, 86a3, 86a4, 86bl, 86b2, 86b3, 86b4, are arranged between the float bodies 22a, 22b and the respective legs 32a, 32b, 32c, 32d, 32e, 32f, 32g and 32h. The fenders can be connected to the structure 2, in particular to the legs 32a, 32b, 32c, 32d, 32e, 32f, 32g and 32h thereof. The fenders can also be connected to the respective float bodies 22a, 22b. The purpose of the fenders is to allow good contact between the float bodies 22a, 22b and the respective legs of the structure 2. This prevents damage to the structure 2 or the float assembly 20.

Furthermore, the float assembly 20 comprises guides 84al, 84a2, 84bl and 84b2. The purpose of the guides is to prevent horizontal movements in the direction of the arrows 88 and 90 between the float assembly 20 and the structure 2. The structure 2 and the float assembly 20, in particular the float bodies 22a, 22b of the float assembly 20, are fixed in the horizontal directions with respect to one another with the aid of the guides 84al, 84a2, 84bl and 84b2. The guides 84al, 84a2, 84bl, 84b2 are arranged in the vicinity of the middle legs 32b, 32c, 32f, 32g and can act on these legs. The guides 84al, 84a2, 84bl, 84b2 can also guide the float bodies 22a, 22b into the correct position with respect to the structure 2 during positioning of the float bodies 22a, 22b with respect to the structure 2.

Connecting members 42a, 42b connect the float bodies 22a, 22b to one another.

Figure 18 shows a view of the connection of leg 32h of the structure 2 and the float body 22b of the float assembly 20. The fender 86b4 is located between the float body 22b and the leg - 24 32h and therefore transmits forces from the float body 22b to the leg 32h.

Figure 19 shows a detailed view of the connection between leg 32g of the structure 2 and the float body 22b. The guide 84b2 extends from the float body 22b in the direction of the leg 32g as far as the side of the leg 32g which is remote from the float body.

Figures 20, 21a, 21b and 21c show a diagrammatic side view of an arm 24a having a first end 26a. The first end 26a comprises two eye devices 86, 87, each adapted to be connected to a respective coupling member 88 having a substantially hook-like form, which coupling members 88 are connected to the structure 2. The eye devices 86, 87 and the coupling members 88 form the connection coupling 38a between the arm 24a and the structure 2. The coupling members 88 have a contact area 92 located at a bottom side thereof. Arm 24b has a similar connection coupling 38b (not shown).

When the arms 24a, 24b are in a substantially horizontal position (shown in fig. 21a), the arms 24a, 24b can rotate freely relative to the structure by a predetermined angle, depicted by arrows 98, 100. This allows for the float bodies 22a, 22b to move relative to the structure 2 in case forces of waves and/or wind are exerted on the float bodies 22a, 22b, thereby reducing peak loads on the structure 2.

When the arms 24a, 24b are in a substantially vertical position (shown in fig. 21c), supports 90 in the form of box girders are moved directly underneath contact area 92 of coupling member 88.

When a positive buoyancy force is to be exerted from the float body 22a, 22b to the structure, each support 90 engages with the contact area 92 of the coupling member 88 and transfers the upward force 94 to the structure 2. Also, a downward force 96 may be transferred to the structure 2, thereby pulling the structure 2 down.

Thus, the coupling 38a can adopt a first operating state in - 25 which a vertical movement of the floating bodies 22a, 22b relative to the structure 2 is allowed, and in which a buoyancy force is not transferred to the structure 2, and a second operating state in which a buoyancy force is transferred to the structure 2, and in which a vertical displacement of the floating bodies 24a, 24b relative to the structure 2 is not allowed.

The legs 32a, 32b of the structure 2 may be extended in an upward direction in order to allow the connection points 28a, 28b to be located higher. - 26

Claims

1. Method for moving a structure (2) which is at least partially underwater, in particular an offshore platform, upwards, the method comprising the steps of: (a)coupling a float assembly (20) to the structure (2), the float assembly (20) comprising at least two float bodies (22a, 22b), which are provided with a variable buoyancy and are coupled to the structure (2) at respective connection points (28a, 28b) by means of associated arms (24a, 24b); (b)bringing the float assembly (20) into a position which is suitable for moving the structure (2) upwards; and (c)increasing the buoyancy of the float bodies (22a, 22b), with the result that the structure (2) is moved upwards.

2. Method according to claim 1, in which in step (b) the float bodies (22a, 22b) are brought to a predetermined depth below the surface (10) of the water.

3. Method according to claim 1 or 2, in which ballast is removed from the float assembly (20) with respect to the structure (2) in such a manner that the structure (2) is held in balance during step (c).

4. Method according to one of claims 1-3, in which in step (a) two float bodies (22a, 22b) are coupled to the structure (2) on opposite sides of the structure (2).

5. Method according to one of claims 1-4, in which the arms (24a, 24b) are connected to the structure (2) by means of respective connection couplings (38).

6. Method according to one of claims 1-5, in which in step (b) the at least two float bodies (22a, 22b) are connected to one another by means of a connecting member (42) for holding the float bodies in a fixed position with respect to one another.

7. Method according to one of claims 1-6, in which each arm (24a, 24b) can pivot about a substantially horizontal axis - 27 - extending through the respective connection point (28a, 28b), and in which in step (b) the arms (24a, 24b) are rotated by a predetermined angle about the respective axes, with the float bodies (22a, 22b) being moved downwards with respect to the structure (2).

8. Method according to one of claims 1-7, in which in step (b) the arms (24a, 24b) are rotated in such a manner with respect to the structure (2) that the float bodies (22a, 22b) substantially bear against the structure (2).

9. Method according to claim 8, in which prior to step (b) forceabsorbing means (86al, 86a2, 86a3, 86a4, 86bl, 86b2, 86b3, 86b4) are arranged between the float assembly (20) and the structure (2).

10. Method according to one of the preceding claims, in which during step (c) the float assembly (20) is held in a predetermined horizontal position with respect to the structure (2) by at least one guide (84al, 84a2, 84bl, 84b2).

11. Method according to one of claims 1-10, in which the connection points (28a, 28b) are located above the surface (10) of the water.

12. Method according to one of claims 1-11, in which the float bodies (22a, 22b) have a positive buoyancy during step (a).

13. Method according to one of claims 1-12, in which in step (b) the buoyancy of the float bodies (22a, 22b) is reduced in order for the float bodies (22a, 22b) to be moved downwards with respect to the structure (2).

14. Method according to one of claims 1-13, further comprising a step (d) in which the structure (2) with the float assembly (20) connected to it is transported from a first location (80) to a second location (82), after which the buoyancy of the float bodies (22a, 22b) is reduced, in order to lower the structure (2).

15. Method according to claim 14, in which the second location (82) has a shallower depth than the first location (80), and in which after the buoyancy has been reduced the float assembly (20) is uncoupled from the structure (2).

16. Method according to one of claims 1-15, in which the structure (2) is at least partially broken up after step (c) has ended.

17. Method according to one of claims 1-16, in which in step (b) two float bodies (22a, 22b) are connected to one another by means of at least two connecting members (42a, 42b), with a closed loop of float bodies (22a, 22b, 22c, 22d) and connecting members (42a, 42b, 42c, 42d) arranged between them being formed around the structure (2).

18. Method according to one of claims 1-17, in which the structure (2) is an offshore platform.

19. Float assembly (20) for moving a structure (2) which is at least partially underwater, in particular an offshore platform, upwards, the float assembly (20) comprising: - at least two float bodies (22a, 22b) provided with a variable buoyancy; - at least two arms (24a, 24b), which are each connected to an associated float body (22a, 22b), it being possible for the float bodies (22a, 22b) to be coupled to the structure at respective connection points (28a, 28b) by means of the arms; and - a connecting member (42) for holding the float bodies (22a, 22b) in a fixed position with respect to one another.

20. Float assembly according to claim 19, in which the float bodies (22a, 22b) are substantially elongate in form, in particular are cylindrical in form or in the shape of a box.

21. Float assembly according to claim 19 or 20, in which the float bodies (22a, 22b) have respective main longitudinal axes - 29 - which extend substantially parallel to the substantially horizontal axes extending through the respective connection points (28a, 28b).

22. Float assembly (20) according to one of claims 19-21, which float assembly (20) is provided with a first connecting member coupling (50al) and a second connecting member coupling (50bl) for coupling the connecting member (42a) to the first float body (22a) and the second float body (22b).

23. Float assembly according to claim 22, in which the first and second connecting member couplings (50al, 50bl) are designed to allow a remotecontrolled connection to be produced between the first and second float bodies (22a, 22b).

24. Float assembly according to one of claims 19-23, in which each arm can be coupled to the structure at at least two connection points (28a, 28b) on the structure.

25. Float assembly according to one of claims 19-24, in which the connection points (28a, 28b) are located above the surface (10) of the water.

26. Float assembly according to one of claims 19-25, further comprising at least two couplings (38a, 38b) for connecting the arms (24a, 24b) to the structure (2).

27. Float assembly according to claim 26, wherein each coupling (38a, 38b) is adapted to allow for a substantially vertical movement of the respective floating bodies (22a, 22b) relative to the structure (2) in a first operating state of the coupling (38a, 38b).

28. Float assembly according to claim 26 or 27, wherein each coupling (38a, 38b) is adapted to prevent a substantially vertical movement of the respective floating bodies (22a, 22b) relative to the structure (2) in a second operating state of the coupling (38a, 38b). -

29. Arm (24a, 24b) suitable for use in the float assembly according to one of claims 19-28.

30. Structure (2), in particular an offshore platform, coupled to a float assembly (20) according to one of claims 19-30.