NO20130829A1 - Apparatus and method for carrying out an operation on at least a partially submerged structure - Google Patents

Abstract

The invention relates to a device (10) for performing an underwater operation on a at least partially submerged structure (40) or in the vicinity of at least partially submerged structure, the device comprising: a foundation (15) comprising at least one coupling (20) to connect the device to the structure and to disconnect the device from the structure,? at least a first articulated movable arm (30) directly or indirectly attached at a proximal end thereof (31) to the foundation, the first movable arm comprising a tool (60) attached to a distal end (29) of the the first movable arm, wherein the tool is movable in a three-dimensional working area about the foundation, wherein the foundation comprises at least one rail (22A, 22B), wherein the movable arm (30) comprises a support carriage (32) which engages the rail for to allow the movable arm to move along the rail.

Description

Scope of the invention

The present invention relates to the area of dismantling structures in marine environments. The present invention further relates to performing other operations on structures in a marine environment.

Background of the invention and prior art

In previous years, many structures have been built on the sea, especially for the exploitation and production of hydrocarbons. These structures are usually partially submerged. Often these structures rest on the seabed and rise above the water level. Structures by the lake have been built for other purposes as well.

At the end of an economic life cycle, such a structure often has to be removed or dismantled. This is often a difficult and expensive process. Often, divers are used to performing underwater operations. This is a great risk to human life. Furthermore, the use of divers at sea is expensive and time-consuming. Sometimes RO Ver is used. However, ROVs also have some limitations. ROVs can only exert limited powers. ROVs are also sensitive to currents, just like divers. Sometimes a purpose-built vessel is used to remove such structures. These vessels are very expensive.

This invention particularly relates to truss structures. A truss comprises many pipe parts which are connected and which form a lattice structure. The beams are usually made of steel. A truss usually comprises a number of beams which are essentially upright and which extend from the foundation of the truss to the top of the truss. The upright beams are usually connected to each other via horizontal and diagonal cross beams

When a truss reaches the end of its economic or technical life, it may be necessary to remove it. This required removal is dependent on local administrative requirements and may be partial or complete.

According to a known method for removing a truss, a heavy lifting vessel is used to support the top part of the truss. Then, the truss is cut at a given distance above the seabed. The part above the cut is lifted and removed by the heavy lifting vessel, and the part below the cut remains in place. A further cutting and lifting operation may be necessary to remove the next part.

The cutting of the truss is usually a time-consuming and expensive operation. Usually a large vessel with a crew is required to perform the cutting. Divers or one or more ROVs and special cutting and lifting equipment are required for the actual cutting. For lifting operations, heavy lifting capacity is required, it is expensive.

The known remote operation also has the disadvantage that bad weather can cause significant delays, which further increases costs. There is a need within the specialist area for more efficient and cost-saving ways of removing the truss structures.

Furthermore, the construction or repair of structures at sea is rather expensive. Also during construction, water operations are necessary. Divers and RO Vere are often used, but have the same disadvantage as when removing or dismantling a structure. Some devices for underwater work other than ROVs have been produced, but none of these devices provide the necessary capacity. There is also a need within the field to improve methods for the construction or repair of structures at sea.

Purpose of the invention

It is an object of the invention to improve operations on structures that are at least partially submerged.

Summary of the invention and its embodiments

A device is provided for performing an underwater operation on an at least partially submerged structure or in the vicinity of an at least partially submerged structure, the device comprising: • a foundation comprising at least one coupling for connecting the device to the structure and disconnecting the device from the structure, • at least one first articulated movable arm directly or indirectly attached at a proximal end thereof to the foundation, the first movable arm comprising a tool attached to a distal end of the first movable arm, wherein the tool is movable in a three-dimensional working area about the foundation, wherein the foundation comprises at least one rail wherein the movable arm comprises a support carriage which engages the rail to allow the movable arm to move along the rail.

With the present invention, a large industrial structure can be dismantled. It is possible to dismantle a truss structure that has a height of, for example, 100 meters or more. The disassembly operation can be carried out essentially independently by the movable arm. The connectors allow the device to be connected and disconnected to and from the structure in a simple way. At least one rail has a length of more than 10 metres, i.e. 10-150 meters depending on the size of the structure.

In one embodiment, the foundation comprises a buoyancy element. The buoyancy element allows easier transport and handling as well as accurate positioning of the device in relation to the structure. The buoyancy element can have a length of more than 10 meters, for example 10-150 meters, depending on the size of the structure.

In one embodiment, the buoyancy element is elongated and the connections are connected to the buoyancy element at different positions along the length of the buoyancy tank. In a further embodiment, the buoyancy tank is elongated and constructed to be mounted to one of the legs of the structure in an orientation that is substantially parallel to the leg.

In one embodiment, the foundation comprises a first rail and a second rail extending parallel to each other at a distance from each other, wherein the support carriage is configured to engage with both the first and second rails. In one embodiment, the at least one rail is mounted to the buoyancy tank. When the buoyancy element forms a structural component to which the connectors and rails are mounted, a solid structure is obtained which can be relatively easily connected to one of the structure's legs.

In one embodiment, the buoyancy element has a longitudinal axis, in which at least one rail extends parallel to the longitudinal axis of the buoyancy element.

In one embodiment, the device comprises a plurality of connectors which are mounted to one side of the buoyancy element, in which at least one rail is mounted to another side of the buoyancy element. This configuration allows substantial freedom of movement for the movable arms.

In one embodiment, the device comprises at least two movable arms, each comprising a support carriage and each movably connected to at least one rail. This configuration allows cooperation between two movable arms. The movable arms can be connected to the same rails. The two movable arms are movable over the rails independently of each other.

In one embodiment, the device comprises two sets of rails and at least one movable arm is connected to each set of rails. The sets can be located on different sides, particularly on opposite sides of the buoyancy element.

In one embodiment, the movable arm comprises a connecting member designed to be connected to a lifting cable extending between a surface vessel and a movable arm.

In one embodiment, the movable arm comprises at least two segments which are connected to each other by a hinge and in which the connecting element is provided close to the hinge.

The present invention further relates to an arrangement comprising:

- a device according to one of claims 1-12,

-a vessel and

- an at least partially submerged structure,

in which the device is connected to the structure, in which the vessel is connected to

the connecting element via a lifting cable, in which the vessel exerts an additional upward force on the movable arm via the lifting cable.

The present invention further relates to a method for performing an underwater operation on an at least partially submerged structure or in the vicinity of an at least partially submerged structure, the method comprises: - providing a device for performing an underwater operation on a at least partially submerged structure or in the vicinity of it an at least partially submerged structure, the device comprising: • a foundation comprising at least one coupling for connecting the device to the structure and disconnecting the device from the structure, • at least a first articulated movable arm directly or indirectly attached at a proximal end thereof to the foundation, the first movable arm comprising a tool attached to a distal end of the first movable arm, wherein the tool is movable in a three-dimensional working area about the foundation, wherein the foundation comprises at least one rail, in which the movable arm comprises a support ecarriage engaging the rail to allow the movable arm to move along the rail, - connecting the device to the structure, in which the foundation of the device is at least partially submerged, - performing an operation with the tool mounted on the distal end of the movable arm.

In one embodiment of the method, the method comprises a buoyancy element, and the method comprises:

-to transport the device to the structure in a liquid state,

- to ballast the buoyancy element with water and thereby lower the device,

- to connect the device to the structure via at least two connections, via at least one connection located under water.

In one embodiment of the method, the buoyancy element is elongated and is connected essentially parallel to a leg of the structure.

In one embodiment of the method, the device comprises at least one rail in which the movable arm comprises a support carriage which engages with the rail and is designed to move along the rail, in which the movable arm moves along the rail from one position to another position, and performs operations at these positions.

In one embodiment of the method comprises two or more movable arms which are movably connected to at least one rail and which perform operations simultaneously

on the structure, especially in cooperatives.

In one embodiment, the method includes removing several parts of the structure at the movable arm, leaving a leg to which the device is connected and support beams that directly support the leg intact.

In one embodiment, the method comprises positioning a vessel in the vicinity of the device and providing one or more connecting lines between the device and the vessel to transmit:

- control signals,

-electric power and/or

-hydraulic fluid from the vessel to the device or vice versa.

In one embodiment, the method comprises; -positioning a vessel comprising a lifting device above the device and providing at least one lifting cable between the lifting device and a connecting element on the movable arm, -performing an operation with the tool mounted on the distal end of the movable arm, wherein during the operation an additional lifting force is applied ( F) from the vessel on the movable arm via the lifting cable.

Overview description of the figures

The former and other features and advantages of the present invention will be understood more fully from the detailed description of exemplary embodiments with reference to the attached drawings.

Figure IA shows a side view of an embodiment of the invention during transport.

Figure IB shows a side view of an embodiment of the invention in Figure IA in connection with the structure. Figure 1C shows a side view of an embodiment of the invention on Figure IA in a next stage of connection with the structure.

Figure 2 shows a side view of an embodiment of the invention during operation.

Figure 3 A shows a cross-section of an embodiment of the invention during operation according to the lines A-A in Figure 2. Figure 3B shows a cross-section of an alternative embodiment of the invention during operation according to the lines A-A in Figure 2. Figure 4A shows a side view of an alternative execution of the invention during operation.

Figure 4B shows a schematic front view of the embodiment in Figure 4A.

Figure 5 A shows a top view of a movable arm according to the invention.

Figure 5B shows a side view of a movable arm according to the invention.

Figure 5C shows a partial view of a movable arm according to the invention comprising a

other tool.

Figure 6 shows a side view of another embodiment of the invention during operation.

Detailed description of the figures

With reference to figures IA, IB, 1C, 2 and 3A, a device 10 according to the invention is shown during transport to the destination. The device is towed by an auxiliary vessel 50 via a line 12. The device 10 comprises a buoyancy element 16. The buoyancy element 16 forms the device 10's foundation 15, for example a central structural component of the device 10. The buoyancy element is elongated and has a circular cross-section.

The device 10 is towed in an essentially upright orientation and most of it is submerged, with a narrow part protruding above the waterline 5. The device 10 can also be towed in another orientation, for example essentially horizontally.

The buoyancy element 16 internally comprises several rooms located along the length of the buoyancy element. The rooms are separated from each other by partitions, as is commonly known in the state of the art. The buoyancy element comprises openings 18 with valves that can be controlled. The valves allow ballasting of the buoyancy element in a controllable manner to lower the device. At least one pump is provided to blow compressed air, nitrogen or other gas into the chambers to remove water when necessary, thereby raising the device 10. When several valves are used, these valves, in a preferred embodiment, can be operated from a single connection point. It is also possible that the device 10 comprises several separate buoyancy elements.

The clamps 20 are connected to the buoyancy element 16. The clamps are positioned at a distance from each other. Two clamps 20 can be provided, but several clamps are also provided. The clamps are configured to connect to the legs of a structure. From this side, the clamps 20 are hydraulically operated. The clamp 20 can also be pneumatically or electrically operated. The clamps 20 are designed to transfer significant forces in three dimensions and significant moments about the x-axis, y-axis and z-axis from the device 10 to the structure 40.

Particularly when the device 10 performs a lifting operation, a downward vertical force is exerted by the device via the connections 20 on the structure 40. If there is a current in the water, the horizontal forces will also be transferred via the clamps 20 to the structure 40.

The device 10 comprises two rails 22A, 22B which are rigidly connected to the buoyancy element 16. The rails 22A, 22B extend parallel to a main axis 24 of the buoyancy element. The rails 22A, 22B can be directly connected to the buoyancy element or can be connected to the buoyancy element via supports.

Two articulated movable arms 30 are connected to the rails 22A, 22B. Each movable arm 30 comprises a carriage 32 which is designed to be moved along the rails 22A, 22B. The carriage includes rollers or brake shoes 36 (eng:skids) which engage with the rail and secure the carriage in two directions across the rail (Y direction and Z direction) while allowing for a direction parallel to the X direction of the rail).

Each movable arm further comprises a turret 34 which is rotatably mounted to the carriage 32 and which extends from the carriage. The turret 34 comprises an internal drive which rotates the turret relative to the carriage. The movable arm will be discussed further below.

The two movable arms 30 are movable over the rails independently of each other. The two movable arms also include carriages 32 which are independently movable from each other in relation to the foundation 15.

From figure IB it can be seen that upon arrival at the structure 40, the device 10 is positioned vertically or substantially parallel to a leg 42 of the structure at the vessel 50. The rooms are controllably flooded to reach the required depth and orientation. From a vertical orientation, the lower clamp is first brought into engagement with the structure. Then the structure is angled to engage the upper frame as well, see Figure 1C.

When the auxiliary vessel 50 arrives at the structure 40, the buoyancy element 16 is ballasted so that it is submerged. Optionally, the element is ballasted so that it rotates. During the connection operation, the device 10 can be connected to the vessel via one or more lines for better control of the movement. The clamps 20 engage one of the legs 42 of the structure and are hydraulically activated to clamp the leg 42 tightly. The device 10 is now rigidly connected in relation to the structure 40. The device can be completely submerged or can be partially above the water line 5.

Figure 2 shows the device 10 during operation. The device 10 is connected to a structure 40 which is to be taken apart. The structure can be a subject work structure where a top page unit has already been removed in a previous operation. The structure comprises legs 42 and cross beams, diagonal beams and other internal beams 44 which are connected at the legs 42 and which provide structural rigidity to the truss structure. The leg typically extends at an angle to the vertical that is less than 10 degrees.

The arms have a three-dimensional operating area in relation to the support carriage 32. Furthermore, the arms 30 have an operating area that is large enough to cover a significant part of the structure 40, due to the possibility of being moved along the rail 22. In this way, the arms can dismantle the structure 40 or in at least dismantle a significant part of the structure 40.

It is possible that the working area 30 of the arms is too small for the entire height of the structure 40 to be covered or for the entire horizontal span of the structure 40 to be covered. In that case, it may be necessary to reposition the device 10 from time to time to change the working area and to allow the tool arms to reach other parts of the structure. This repositioning may involve connection of the device 10 to another leg 42.

During operation, the device 10 is connected to the auxiliary vessel 50 via line 75. These lines may include power cables to provide electrical power, a hydraulic cable to provide hydraulic power, control lines to control various actuators, pumps and valves. Lines for transmitting signals from sensors can also be provided. During operation, the auxiliary vessels may be in the vicinity of the device 10. In the event of a storm, the connecting lines 75 may be disconnected and the auxiliary vessel 50 may be moved to a safer area. The lines can be attached to each other to form an umbilical cord/connecting line ("umbilical").

The two parallel rails 22A, 22B are shown in Figure 3. The support carriage 32 comprises braking elements 36 ("skidding elements") which are connected to the rails. The brake elements 36 engage both the front and rear of each rail 22A, 22B and also the side of the rails.

A rack 70 is connected to the buoyancy element between the rails 22A, 22B and extends parallel to the rails 22A, 22B. The support carriage 32 comprises a rack s72 which is connected to a rack 74. The rack and rack allows the movable arm to move in a controlled manner along the rails 22A, 22B. Other ways of moving the support carriage 32 in a controlled manner relative to the buoyancy element are achievable. For example, in another embodiment, the carriage comprises rollers instead of brake elements. One or more rollers can be driven by a drive 74.

The rails 22A, 22B are connected to a different side of the buoyancy element 16 than the clamps 20.

The movable arm 30 is connected at a proximal end to the turret via a first hinge 35. The movable arm comprises a first segment 37 and a second segment 38, which are connected via a second hinge 39. A tool 60 is connected to the second segment 38 via a third hinge 62 at a distal end 29 of the arm 30. The tool 60 is itself rotatable relative to a holder 63 via which the tool 60 is connected to the hinge 62. The arm 30 is equipped with actuators to allow controlled movement of the arm, which are discussed in more detail below.

The tool 60 is a cutting tool and can be a hydraulic cutter, a saw, a flame cutting tool, a welding tool or a lasing device. The tool can also be a gripping tool, drilling tool or another tool. The tool has a three-dimensional working area about the carriage 32. The tool 60 is rotatable about three independent axes of rotation relative to the distal end 29, resulting in six degrees of freedom relative to the carriage 32.

A control unit is provided on board the auxiliary vessel to control the device 10. One or more operators may be present to control the device, or the device may be fully automated.

In one embodiment, the device 10 comprises a movable arm 30 comprising several sensors which are used to determine a position of the movable arm and of the processing tool relative to the structure. These sensors can be ultrasonic sensors, video cameras and or contactors that register engagement of the moving arm with the structure. Other sensors may be provided, such as a current sensor to measure the current or a weight sensor to measure the weight of a part of the structure 40 that is lifted by the arm 30.

The clamps 20 comprise two clamp halves 21 A, 21B which are hydraulically operated to be moved away from each other or to be moved towards each other, for example to be moved between an open position and a closed position. The clamps are mounted to the buoyancy element 16 via supports 26.

Another embodiment shown in figure 3b has two sets of rails 22A, 22B on opposite sides of the buoyancy element 16. A movable arm 30 is connected to a pair of rails 22A, 22B and another movable arm 30 is connected to another pair of rails 22A, 22B. This embodiment has an advantageous weight distribution in that the weight is balanced with respect to a longitudinal axis 24 of the buoyancy tank, and also with respect to a central plane 49 extending through the longitudinal axis 24 and the longitudinal axis 25 of the leg 24.

Furthermore, two movable arms can pass each other when moving along the foundation

15 length, because they are connected to different rails.

Figures 4A and 4B show another embodiment of the device 10 which does not

comprises a buoyancy tank 16. The device comprises a set of interconnected parallel rails 22A, 22B. The rails are connected via cross beams 23 and are positioned at a distance of 90 from each other. The device 10 further comprises four clamps 20, but a different number of clamps is also possible. The clamps are connected to the cross beams 23. The movable arms 30 including the respective carriage are essentially the same as for the embodiment in Figures 1-3.

Before the operation, the device 10 is transported to the structure 40 by an auxiliary vessel 50. The device 10 is lowered via lines or cables by a crane or other lifting device on the vessel 50. The device 10 is positioned parallel to and in engagement with a leg 42 of the structure 40 The clamps 20 are hydraulically operated to grip the leg 42 firmly.

An embodiment of the movable arm 30 is shown in Figures 5A and 5B. The movable arm 30 comprises a carriage comprising brake elements 36. A turret 34 is mounted to the upper side of the carriage 32. The first segment 37 is connected via the hinge 35 to the turret 34. A drive is provided inside the carriage to rotate the turret relative to the carriage 32. An actuator 76 in the form of a hydraulic cylinder is provided to rotate the first segment relative to the turret. The actuator 76 extends between pivot points 79 on the turret 34 and the first arm 37, respectively.

Another hinge 39 connects the second segment 38 to the first segment 37. Another actuator 77 is provided to rotate the second segment 38 relative to the first segment. The actuator 77 extends between a pivot point 79 on the first segment and a mechanism 81 comprising several axes and beams which function as a lever arm.

The tool 60 in figures 5A, 5B is rotatably mounted on holder 63, which is in turn connected to the third hinge 62.

A connecting element 82 is provided on the first segment near the first hinge 39, the purpose of which is discussed in connection with Figures 6, 7A, 7B, 7C.

The support carriage 32 comprises two sliding or rolling elements 36 which are in engagement with the first rail 22A at a distance 47 from each other, and at least two sliding or rolling elements 36 in engagement with the second rail 22B at a distance 47 from each other.

A gripping tool is shown in Figure 5C. One arm 30 will typically have a cutting tool and another arm 30 of the device 10 will have a gripping tool. In a typical operation, one arm grips a portion of the structure to be removed with a gripping tool et, and the other arm 30 cuts the portion free from the remainder of the structure 40 with a cutting tool et 60.

Another embodiment is shown in Figure 6. The device comprises a connection point 82 where a lifting cable 84 can be connected to the arm 30. The lifting cable 84 extends between the auxiliary vessel 50 and the movable arm 30. In the figure, the connection point 82 is provided near the first hinge 39 between the first the segment 37 and the second segment 38, but it may be elsewhere as well.

The lifting cable 84 can be connected to a hoisting device 85 on the vessel 50. The hoisting device comprises a winch 87 and a boom 86 which extends outside the vessel. In operation, lifting cable 84 can provide an additional lifting force F, so that the lifting capacity of arm 30 is increased. The lifting cable 84 can also be used in the designs in figures 1, 2, 3, 4, 5, 5A, 5B, 5C. The hoisting device 87 is configured to take in the cable when the tool 60 is to be moved upwards, and to uncoil the cable when the tool 60 is to be moved downwards. The lifting device 87 comprises a heave compensation as is known in the state of the art.

In another embodiment, the connecting element 82 is provided on the second segment 38, particularly near the third hinge.

It will be obvious to the person skilled in the art that details and the arrangement of the parts can be varied over a considerable range without leaving the scope of protection of the invention as defined by the claims. The device according to the invention can in particular be used for operations other than disassembly operations. Maintenance work, cleaning operations, and other operations can also be carried out.

Furthermore, the person skilled in the art will understand that the present invention can be used to mount a structure under water.

Claims (21)

1. Device (10) for performing an underwater operation on an at least partially submerged structure (40) or in the vicinity of at least partially submerged structure, the device comprises: • a foundation (15) comprising at least one coupling (20 ) to connect the device to the structure and disconnect the device from the structure, • at least a first articulated movable arm (30) directly or indirectly attached at a proximal end thereof (31) to the foundation, the first movable arm comprising a tool ( 60) which is attached to a distal end (29) of the first movable arm, in which the tool is movable in a three-dimensional working area about the foundation, wherein the foundation comprises at least one rail (22A, 22B), wherein the movable arm (30) comprises a support carriage (32) which engages the rail to allow the movable arm to move along the rail. 2. Device according to claim 1, in which the foundation (15) comprises a buoyancy element (16). 3. Device according to claim 2, in which the buoyancy element is elongated and in which the couplings (20) are attached to the buoyancy element at different positions along the length of the buoyancy tank. 4. Device according to one of the preceding claims, in which the buoyancy element (16) is elongate and designed to be mounted to the legs (42) of the structure (40) in an orientation that is essentially parallel to the leg (42) 5. Device according to one of the preceding claims, in which the foundation comprises a first rail (22A) and a second rail (22B) which extend parallel to each other at a distance (90) from each other, in which the support carriage (32) is designed to engage both the first and second rails. 6. Device according to claim 5, in which at least one rail (22A, 22B) is mounted to the buoyancy element (16). 7. Device according to one of claims 4-6, in which the buoyancy element has a longitudinal direction (24), in which the at least one rail (22A, 22B) extends parallel to the longitudinal axis of the buoyancy element (24). 8. Device according to one of claims 3-7, comprising a plurality of connectors (20) which are mounted to one side of the buoyancy element, in which at least one rail (22A, 22B) is mounted to another side of the buoyancy element. 9. Device according to one of the preceding claims, comprising at least two movable arms (30), each comprising a support carriage (32) and each movable fixed to at least one rail, the at least two movable arms being movable over the rail independently each other. 10. Device according to claim 9, comprising two rail sets, and where at least one movable arm (30) is movably attached to each rail set. 11. Device according to one of the preceding claims, wherein at least one movable arm comprises a connecting part (82) designed to be connected to a lifting cable (84) extending between a surface vessel (50) and the movable arm. 12. Device according to claim 11, in which the movable arm comprises two segments (37, 38) attached to each other by a hinge (39) and in which the connecting part (82) is provided close to the hinge. 13. Arrangement_comprising: - a device according to one of claims 1-12, - a vessel (50) and - an at least partially submerged structure (40), wherein the device is connected to the structure (40), wherein the vessel is connected to the connecting part (82) via a lifting cable (84), wherein the vessel exerts a force directed upwards on the movable arm via the lifting cable. 14. Method for performing an underwater operation on an at least partially submerged structure (40) or in the vicinity of an at least partially submerged structure, the method comprises: - providing a device (10) for performing an underwater operation on an at least partially submerged structure (40) or in the vicinity of an at least partially submerged structure, the device comprises: • a foundation (15) comprising at least one coupling (20) for connecting the device to the structure and disconnecting the device from the structure, • at least one first articulated movable arm (30) directly or indirectly attached at a proximal end thereof (31) to the foundation, the first movable arm comprising a tool (60) attached to a distal end (29 ) of the first movable arm, in which the tool is movable in a three-dimensional working area about the foundation, wherein the foundation comprises at least one rail (22A, 22B), wherein the movable arm (30) comprises a support carriage (32) which engages the rail to allow the movable arm to be moved along the rail, -connecting the device to the structure, in which the foundation of the device is at least partially submerged, - performing an operation with the tool mounted on the distal end of the movable arm. 15. Method according to claim 14, in which the device comprises a buoyancy element (16), and in which the method comprises: - transporting the device (10) to the structure in a liquid state, - ballasting the buoyancy element (10) with water, thereby lowering the device, - to connect the device to the structure via at least two connections (20), in which at least one connection (20) is underwater. 16. Method according to claim 14 or 15, in which the buoyancy element is elongated and is connected essentially parallel to a leg (42) of the structure. 17. Method according to one of claims 14-16, in which the device comprises at least one rail (40) in which the movable arm comprises a support carriage (32) which engages with the rail and is designed to be moved along the rail, in which the the movable arm moves along the rail from one position to another position, and performs operations at these positions. 18. Method according to one of claims 14-17, in which the device comprises two or more movable arms (30) which are movable connected to at least one rail and which perform operations simultaneously on the structure, particularly in cooperation. 19. Method according to one of claims 14-18, comprising removing several parts of the structure at the movable arm and leaving a leg (42) to which the device is connected and support beams that directly support the leg (42) intact. 20. Method according to one of the preceding claims 14-19, comprising positioning a vessel (50) in the vicinity of the device and providing one or more connecting lines (75) between the device and the vessel to transmit: - control signals, - electrical power and /or -hydraulic fluid from the vessel to the device or vice versa. 21. Method according to one of the preceding claims 14-19, comprising positioning a vessel comprising a lifting device (87) above the device (10) and providing at least one lifting cable (84) between the lifting device and a connecting element (82) on the movable arm, - performing an operation with the tool (60) mounted on the distal end of the movable arm, wherein during the operation an additional lifting force (F) is exerted from the vessel on the movable arm via the lifting cable.