NL2009005C2

NL2009005C2 - Device and method for performing an operation on an at least partially submerged structure.

Info

Publication number: NL2009005C2
Application number: NL2009005A
Authority: NL
Inventors: Robert Reinder Narold
Original assignee: Heerema Marine Contractors Nl
Priority date: 2012-06-14
Filing date: 2012-06-14
Publication date: 2013-12-17
Also published as: NO20130829A1; GB2504605A; US20130336724A1; GB201310366D0

Description

P31198NL00/WHA

Title: Device and method for performing an operation on an at least partially submerged structure

Field of the invention

The present invention relates to the field of disassembling structures in a marine environment. The present invention further related to performing other operations on structures in a marine environment.

5

Background of the invention and prior art

In the past years, many structures have been built at sea, in particular for exploration and production of hydrocarbons. These structures are generally partially submerged. Often, these structures rest on a seabed and protrude above the water level. Structures at sea 10 have been built for other purposes as well.

At the end of the economic lifespan, such a structure often needs to be removed or disassembled. This is often a difficult and costly process. Often, divers are used to perform under water operations. This brings along great risks to human life. Moreover, the use of 15 divers at sea is costly, and time consuming. Sometimes, ROV’s are used. However, ROV’s also have certain limitations. ROV’s can only exert limited forces. ROV’s further are sensitive to currents, just as divers. Sometimes a special-purpose vessel is used to remove such structures. These vessels are very expensive.

20 The present invention relates in particular to jacket structures. A jacket comprises many tubulars which are interconnected and form a lattice structure. The beams are generally made from steel. A jacket generally comprises a number of beams which are substantially upstanding and which extend from a base of the jacket to a top of the jacket. The upstanding beams are generally connected to one another via horizontal and diagonal 25 cross-beams.

Once a jacket reaches the end of its economic or technical lifespan, it may need to be removed. The required removal is dependent on local administrative requirements and can be in part or in full.

In a known method of removing a jacket, a heavy lift vessel is used to support a top part of the jacket. Subsequently, the jacket is cut at a certain distance above the seabed.

30 -2-

The section above the cut is lifted and removed with the heavy lift vessel, and the section below the cut remains on location. A further cutting and lifting operation may be required to remove a next part.

5 The cutting of the jacket generally is a time consuming and expensive operation.

Generally, a large vessel with a crew is required in order to perform the cutting. Divers or one or more ROV’s and special cutting and lifting equipment are required for the actual cutting itself. For the lifting operation, heavy lift capability is required, which is expensive.

10 The known removal operation has a further disadvantage in that bad weather can cause significant delays, which further increase the costs. There is a need in the field of the art for more efficient and cost-effective way of removing jacket structures.

Further, construction or repair of structures at sea is also rather costly. Also during 15 construction, underwater operations are necessary. Divers and ROV’s are often used, but have the same disadvantages as during the removal or disassembling of a structure. Some apparatuses for underwater work other than ROV’s have been conceived, but none of these apparatuses provide the capabilities needed. There is also a need in the art for improved methods of constructing or repairing structures at sea.

20

Object of the Invention

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

25 Summary of the invention and embodiments A device is provided for performing an underwater operation on an at least partially submerged structure or in the vicinity of the at least partially submerged structure, the device comprising: o a base comprising at least one coupling for coupling the device to the 30 structure and uncoupling the device from the structure, o at least a first articulated movable arm directly or indirectly connected at a proximal end thereof to the base, the first movable arm comprising a tool connected to a distal end of said first movable arm, wherein the tool is movable in a three dimensional working range about the base, 35 wherein the base comprises at least one rail, wherein the movable arm comprises a support carriage which engages with the rail in order to allow the movable arm to move along said rail.

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With the present invention, a large jacket structure can be disassembled. It is possible to disassemble a jacket structure having a height of for instance 100 meter or more. The disassembling operation can be performed substantially independently by the movable arm. The couplings allow coupling and uncoupling of the device to and from the structure in 5 a simple manner. The at least one rail may have a length of more than 10 meter, i.e. 10-150 meters, depending on the size of the structure.

In an embodiment, the base comprises a buoyancy element. The buoyancy element allows easier transportation and handling as well as accurate positioning of the device 10 relative to the structure. The buoyancy element may have a length of more than 10 meter, i.e. 10-150 meters, depending on the size of the structure.

In an embodiment, the buoyancy element is elongate and wherein the couplings are connected to the buoyancy element at different positions along the length of the buoyancy 15 tank. In a further embodiment, the buoyancy element is elongate and constructed to be mounted to a leg of the structure in an orientation which is substantially parallel to said leg.

In an embodiment, the base comprises a first rail and a second rail which extend parallel to one another at a distance from one another, wherein the support carriage is 20 constructed to engage both the first and the second rail. In an embodiment, the at least one rail is mounted to the buoyancy element. When the buoyancy element forms a structural component to which the couplings and the rails are mounted a sturdy structure is obtained which can be coupled to a leg of a structure relatively easy.

25 In an embodiment, the buoyancy element has a longitudinal axis, wherein the at least one rail extends parallel to the longitudinal axis of the buoyancy element.

In an embodiment, the device comprises a plurality of couplings which are mounted to one side of the buoyancy element, wherein the at least one rail is mounted to a different 30 side of the buoyancy element. This configuration allows a substantial freedom of movement for the movable arms.

In an embodiment, the device comprises at least two movable arms, each comprising a support carriage and each being movably connected to the at least one rail. This 35 configuration allows cooperation between two movable arms. The movable arms may be connected to the same rails. The two movable arms are movable over the rails independently from one another.

-4- ln an embodiment, the device comprises two sets of rails, and wherein at least one movable arm is movably connected to each set of rails. The sets may be located on different sides, in particular opposing sides of the buoyancy element.

5 In an embodiment, the movable arm comprises a connection member constructed to be connected to a lifting cable which extends between a surface vessel and the movable arm.

In an embodiment, the movable arm comprises at least two segments connected to 10 one another by a hinge and wherein the connection member is provided near said hinge.

The present invention further relates to an assembly comprising: - a device according to any of claims 1 - 12, - a vessel, and 15 - an at least partially submerged structure, wherein the device is connected to the structure, wherein the vessel is connected to the connection member via a lifting cable, wherein the vessel exerts an additional upward force on the movable arm via the lifting cable.

20 The present invention further relates to a method of performing an under water operation on an at least partially submerged structure or in the vicinity of the at least partially submerged structure, the method comprising: providing a device for performing an under water operation on an at least partially submerged structure or in the vicinity of the at least partially submerged structure, 25 the device comprising: o a base comprising at least one coupling for coupling the device to the structure and uncoupling the device from the structure, o at least a first articulated movable arm directly or indirectly connected at a proximal end thereof to the base, the first movable arm comprising a tool 30 connected to a distal end of said first movable arm, wherein the tool is movable in a three dimensional working range about the base, wherein the base comprises at least one rail, wherein the movable arm comprises a support carriage which engages with the rail in order to allow the movable arm to move along said rail, 35 - connecting said device to the structure, wherein the base of the device is at least partially submerged, -5- performing an operation with the tool mounted on the distal end of the movable arm.

In an embodiment of the method, the device comprises a buoyancy element, and 5 wherein the method comprises: transporting the device to the structure in a floating condition, ballasting the buoyancy element with water, thereby lowering the device, connecting the device to the structure via at least two couplings, wherein at least one coupling is located underwater.

10

In an embodiment of the method, the buoyancy element is elongate and is connected substantially parallel to a leg of the structure.

In an embodiment of the method, the device comprises at least one rail, wherein the 15 movable arm comprises a support carriage which engages with the rail and is constructed to move along said rail, wherein the movable arm travels along said rail from one position to another position, and performs operations at said positions.

In an embodiment of the method, the device comprises two or more movable arms 20 which are movably connected to the at least one rail and which simultaneously perform operations on the structure, in particular in cooperation.

In an embodiment, the method comprises removing several parts of the structure by the movable arm, and leaving a leg to which the device is connected and supporting beams 25 which directly support the leg intact.

In an embodiment, the method comprises positioning a vessel in the vicinity of to the device and providing one or more connection lines between the device and the vessel for conveying: 30 - control signals, - electric power, and/or hydraulic fluids from the vessel to the device or vice versa.

35 In an embodiment, the method comprises: -6- positioning a vessel comprising a hoisting device above to the device and providing at least one lifting cable between the hoisting device and a connection member on the movable arm, - performing an operation with the tool mounted on the distal end of the movable 5 arm, wherein during the operation an additional lifting force F is exerted on the movable arm via the lifting cable by the vessel.

Brief description of the figures

The previous and other features and advantages of the present invention will be 10 more fully understood from the following detailed description of exemplary embodiments with reference to the attached drawings.

Figure 1A shows a side view of an embodiment of the invention during transport. Figure 1B shows a side view of the embodiment of figure 1A during coupling with the structure.

15 Figure 1C shows a side view of the embodiment of figure 1A in a next step of the coupling with the structure.

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

20 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 another embodiment of the invention during operation.

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

25 Figure 5A 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 another tool.

Figure 6 shows a side view of another embodiment of the invention during operation. 30

Detailed description of the figures

With reference to figures 1A, 1B, 1C, 2 and 3A, a device 10 according to the invention is shown during its transport to a target location. The device 10 is being towed by an auxiliary vessel 50 via a line 12. The device 10 comprises a buoyancy element 16. The 35 buoyancy element 16 forms a base 15 of the device 10, i.e. a central structural component of the device 10. The buoyancy element is elongate and has a circular cross-section.

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The devicelO is being towed in a substantially upright orientation and is submerged for the greater part, with a small part protruding above the water line 5. The device 10 may also be towed in a different orientation, for instance substantially horizontal.

5 The buoyancy element 16 internally comprises multiple compartments which are located along the length of the buoyancy element. The compartments are separated from one another by bulkheads, as is commonly known in the prior art. The buoyancy element comprises openings 18 with controllable valves. The valves allow ballasting of the buoyancy element in a controlled manner in order to submerge the device. At least one pump is 10 provided for blowing compressed air, nitrogen or another gas into the compartments for removing the water when required, thereby raising the device 10. When multiple valves are used, in a preferred embodiment, these valves can be operated from a single connection point. It is also possible that the device 10 comprises several separate buoyancy elements.

15 Clamps 20 are connected to the buoyancy element 16. The clamps are positioned at a distance from one another. Two clamps 20 may be provided, but more clamps may also be provided. The clamps are configured to be connected to a leg of a structure. To this end, the clamps 20 are hydraulically operated. The clamps 20 may also be pneumatically or electrically operated. The clamps 20 are constructed to transfer substantial forces in three 20 dimensions and substantial moments about the X-axis, Y-axis and Z-axis from the device 10 to the structure 40.

In particular when the device 10 performs a lifting operation, a downward vertical force is exerted by the device 10 via the couplings 20 onto the structure 40. If a current 25 exists in the water, 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 30 element 16. The rails 22A, 22B may be connected directly to the buoyancy element or may be connected to the buoyancy element via struts.

Two articulated movable arms 30 are connected to the rails 22A, 22B. Each movable arm 30 comprises a carriage 32 constructed to move along the rails 22A, 22B. The carriage 35 comprises rollers or skids 36 which engage the rail and fixate the carriage in two directions transverse to the rail (Y-direction and Z-direction) while allowing a movement parallel to the rail X-direction).

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

5

The two movable arms 30 are movable over the rails independently from one another. The two movable arms 30 including the carriages 32 are also independently movable from one another relative to the base 15.

10 Turning to figure 1B, upon arrival at the structure 40, the device 10 is positioned vertical or substantially parallel to a leg 42 of the structure by the vessel 50. The compartments are controllably flooded to reach the required depth and orientation. From a vertical orientation, the lower clamp is engaged on the structure first. Then the structure is tilted in order to engage the upper clamp as well, see figure 1C.

15

When the auxiliary vessel 50 arrives at the structure 40, the buoyancy element 16 is ballasted so that it submerges. Optionally the element is ballasted such that it rotates.

During the connection operation, the device 10 may remain connected to the vessel via one or more lines for more control of the movement. The clamps 20 engage a leg 42 of the 20 structure 40 and are hydraulically energized to firmly clamp the leg 42. The device is 10 now rigidly coupled relative to the structure 40. The device may be completely submerged or may be partially above the water line 5.

In figure 2, the device 10 is shown in operation. The device 10 is connected to a 25 structure 40 which needs to be dismantled. The structure may be jacket structure of which a top side unit has already been removed in an earlier operation. The structure comprises legs 42 and cross-beams, diagonal beams and other internal beams 44 which interconnect the legs 42 and provide structural rigidity to the jacket structure. The leg typically extends at an angle to the vertical of less than 10 degrees 30

The arms 30 have a three-dimensional operating range relative to the support carriage 32. Further, due to the ability to travel along the rail 22, the arms 30 have an operating range which is large enough to cover a substantial portion of the structure 40. In this way, the arms 40 can disassemble the structure 40 or at least disassemble a substantial 35 part of the structure 40.

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It is possible that the working range of the arms 30 is too small to cover the entire height of the structure 40 or to cover the entire horizontal span of the structure 40. In that case, the device 10 may need to be repositioned from time to time to change the working range and allow the tools of the arms to reach other portions of the structure. The 5 repositioning may involve connecting the device 10 to another leg 42.

During operation, the device 10 is connected to the auxiliary vessel 50 via lines 75. These lines 75 may comprise a power cable for providing electrical power, a hydraulic cable for providing hydraulic power, control lines for controlling the various actuators, pumps and 10 valves. Also lines for relaying signals from sensors may be provided. During operation, the auxiliary vessel stays in the vicinity of the device 10. In case of storm, the connection lines 75 may be disconnected and the auxiliary vessel50 may be moved to a safer area. The lines may be attached to one another to form an umbilical.

15 Turning to figure 3, the two parallel rails 22A, 22B are shown. The support carriage 32 comprises skidding elements 36 which are connected to the rails. The skidding elements 36 engage both the front side and the back side of each rail 22A, 22B, and also the side of the rails.

20 A rack 70 is connected to the buoyancy element 16 between the rails 22A, 22B and extends parallel to the rails 22A, 22B. The support carriage 32 comprises a pinion 72 which is connected to a drive 74. The rack and pinion allows the movable arm to travel in a controlled manner along the rails 22A, 22B. Other ways of controllably moving the support carriage 32 of the movable arm relative to the buoyancy unit are conceivable. For instance, 25 in another embodiment, the carriage comprises rollers instead of skidding elements. One or more of the rollers may 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.

30

The movable arm 30 is connected at a proximal end 31 to the turret 34 via a first hinge 35. The movable arm comprises a first segment 37 and a second segment 38, which are interconnected 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 itself is rotatable relative to 35 a socket 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.

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The tool 60 is a cutting tool, and can be a hydraulic cutter, a saw, a flame cutting tool, a welding tool, or a laser device. The tool may also be a gripping tool, drilling tool or a different tool. The tool has a three dimensional working range about the carriage 32. The tool 60 is rotatable about three independent axes of rotation relative to the distal end 29, 5 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 10 may be fully automated.

10

In an embodiment, the device 10 comprising the movable arm 30 comprises several sensors which are used for determining a position of the movable arm and of the processing tool relative to the structure. These sensors may be ultrasonic sensors, video cameras and/or touch sensors which register an engagement of the movable arm with the structure. 15 Other sensors may also be provided, such as a current sensor for measuring the current or a weight sensor for measuring the weight of a part of the structure 40 which is lifted by the arm 30.

The clamps 20 comprise two clamp halves 21 A, 21B which are hydraulically operated 20 to move away from one another or to move toward one another, i.e. to move between an open position and a closed position. The clamps are mounted to the buoyancy element 16 via struts 26.

Turning to figure 3b, another embodiment is shown having two sets of rails 22A, 22B 25 on opposing sides of the buoyancy element 16. One movable arm 30 is connected to one pair of rails 22A, 22B and another movable arm 30 is connected to the other pair of rails 22A, 22B. This embodiment has an advantageous weight distribution in the sense that the weight is balanced with respect to a longitudinal axis 24 of the buoyancy tank, and also with respect to a midplane 49 which extends through the longitudinal axis 24 and the longitudinal 30 axis 25 of the leg 24.

Further the two movable arms can pass one another when travelling along the length of the base 15, because they are connected to different rails.

35 Turning to figures 4A and 4B, another embodiment of the device 10 is shown which does not comprise a buoyancy tank 16. The device comprises a set of interconnected parallel rails 22A, 22B. The rails are interconnected via cross-beams 23 and are positioned -11 - at a distance 90 from one another. The device 10 further comprises four clamps 20, but another number of clamps is also possible. The clamps 20 are connected to the crossbeams 23. The movable arms 30 including the respective carriage 32 are substantially the same as for the embodiment of figures 1-3.

5

Prior to 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 another hoisting 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 firmly grip the leg 42.

10

Turning to figures 5A and 5B, an embodiment of the movable arm 30 is shown. The movable arm 30 comprises a carriage 32 which comprises skidding elements 36. A turret 34 is mounted to a top side 33 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 15 the carriage 32. An actuator 76 in the form of a hydraulic cylinder is provided to rotate the first segment relative to the turret 34. The actuator 76 extend between pivots 79 on respectively the turret 34 and the first arm 37

Another hinge 39 connects the second segment 38 to the first segment 37. Another 20 actuator 77 is provided to rotate the second segment 38 segment relative to the first segment. The actuator 77 extends between a pivot 79 on the first segment and a mechanism 81 which comprises multiple axes and beams and which acts as a lever.

The tool 60 in figures 5A, 5B is rotatably mounted to the socket 63, which in turn is 25 connected to the third hinge 62.

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

30 The support carriage 32 comprises two sliding or rolling elements 36 which are engaged with the first rail 22A at a distance 47 from on another, and at least two sliding or rolling elements 36 engaged with the second rail 22B at a distance 47 from one another.

Turning to figure 5C, a gripping tool 60 is shown. Typically, one arm 30 will have a 35 cutting tool and another arm 30 of the device 10 will have a gripping tool. In a typical operation, one arm grips a part of the structure which is to be removed with the gripping tool, - 12- and the other arm 30 cuts the part loose from the remainder of the structure 40 with the cutting tool 60.

Turning to figure 6 another embodiment is shown. The device 10 comprises a 5 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 segment 37 and the second segment 38 but it may be elsewhere as well.

10 The lifting cable 84 may be connected to a hoisting device 85 on the vessel 50. The hoisting device comprises a winch 87 and a boom 86 which extends outboard of the vessel. In operation, the lifting cable 84 can provide an additional lifting force F, substantially increasing the lifting capacity of the arm 30. The lifting cable 84 can also be used in the embodiments of figures 1, 2, 3, 4, 5A, 5B, 5C. The hoisting device 87 is configured to take in 15 cable when the tool 60 needs to be moved upward, and to spool out cable when the tool 60 needs to be moved downward. The hoisting device 87 comprises a heave compensation device as is known in the priori art.

In another embodiment, the connection member 82 is provided on the second 20 segment 38, in particular near the third hinge 62.

It will be obvious to a person skilled in the art that the details and the arrangement of the parts may be varied over considerable range without departing from the scope of the invention as defined by the claims. In particular, the device according to the invention may 25 be used for different operations than a disassembling operation. Maintenance work, cleaning operations, and other operations may also be performed.

Further, the skilled person will understand that the present invention may also be used for assembling a structure underwater.

30

Claims

Device (10) for performing an underwater operation on an at least partially submerged structure (40) or in a vicinity of the at least partially submerged structure, the device comprising: o a base (15) comprising at least one coupling ( 20) for coupling the device to the structure and uncoupling the device from the structure, at least one articulated movable arm (30) connected directly or indirectly to a base at a proximal end (31) thereof, wherein the first movable arm comprises a tool (60) connected to a distal end (29) of said first movable arm, the tool being movable in a three-dimensional working area around the base, the base having at least one rail (22A) , 22B), wherein the movable arm (30) comprises a support carriage (32) which engages the rail to cause the movable arm to move along said rail. 15

Device according to claim 1, wherein the base (15) comprises a floating element (16).

3. Device as claimed in claim 2, wherein the floating element is elongated and wherein at different positions along the length of the floating element couplings (20) are connected to the floating element.

Device according to any of the preceding claims, wherein the floating element (16) is elongated and adapted to be mounted on a leg (42) of the structure (40) in an orientation that is substantially parallel to said leg. 25

Device according to any of the preceding claims, wherein the base comprises a first rail (22A) and a second rail (22B) extending parallel to each other at a distance (90) from one another, the support carriage (32) being constructed to engage on both the first and the second rail. 30

The device of claim 5, wherein the at least one rail (22A, 22B) is mounted on the driving element (16).

Device according to one of claims 4 to 6, wherein the floating element has a longitudinal axis (24), the at least one rail (22A, 22B) being parallel to the longitudinal axis (24) of the floating element extends.

Device according to one of claims 3 to 7, comprising a plurality of couplings (20) mounted on one side of the driving element, the at least one rail (22A, 22B) being mounted on another side of the driving element.

9. Device as claimed in any of the foregoing claims, comprising at least two movable arms (30), wherein each arm comprises a support carriage (32) and is movably connected to the at least one rail, the at least two movable arms being independent of be movable together over said rail.

Device according to claim 9, comprising two sets of rails, and wherein at least one movable arm (30) is movably connected to a respective set of rails. 15

The device of any preceding claim, wherein the at least one movable arm comprises a connector (82) constructed to be connected to a hoisting cable (84) extending between a surface vessel (50) and the movable arm. 20

The device of claim 11, wherein the movable arm comprises at least two segments (37, 38) connected to each other via a hinge (39) and wherein the connecting member (82) is provided adjacent said hinge.

An assembly comprising: a device according to any of claims 1 to 12, a vessel (50), and an at least partially submerged structure (40), the device being connected to the structure (40), the vessel 30 being connected with the connecting member (82) via a hoisting cable (84), wherein the vessel exerts an upward force on the movable arm via the hoisting cable.

A method for performing an underwater operation on an at least partially submerged structure (40) or in the vicinity of the at least partially submerged structure, the method comprising: providing a device (10) for performing of an underwater operation on an at least partially submerged structure (40), the device comprising: o a base (15) comprising at least one coupling (20) for coupling the device to the structure and uncoupling the device from the construction, at least one articulated movable arm (30) connected directly or indirectly at a proximal end (31) thereof to the base, the first movable arm comprising a tool (60) connected to a distal 10 (29) ) end of said first movable arm, the tool being movable in a three-dimensional working area around the base, the base comprising at least one rail (22A, 22B), wherein the movable arm (30) comprises a support carriage (32) that engages the rail to cause the movable arm to move along said rail, connecting said device to the structure, the base of the device at least partially submerged is performing an operation with the tool mounted at the distal end of the movable arm.

A method according to claim 14, wherein the device comprises a floating element (16), and wherein the method comprises: transporting the device (10) to the structure in a floating state, filling water with the floating element (16), the device moving downwards, connecting the device to the structure via at least two couplings (20), at least one coupling (20) being underwater.

A method according to claim 14 or 15, wherein the floating element is elongated and is connected to the leg substantially parallel to a leg (42) of the structure.

17. Method as claimed in any of the claims 14-16, wherein the device comprises at least one rail (40), wherein the movable arm comprises a support carriage (32) which engages the rail and is designed to move along said rail, wherein the movable arm moves along the rail from one position to the other position, and performs operations at said positions. -16-

A method according to any of claims 14-17, wherein the device comprises two or more movable arms (30) which are movably connected to the at least one rail and which perform simultaneous operations on the structures, in particular in cooperation. 5

A method according to any of claims 14-18, comprising removing different parts of the structure by the movable arm, and leaving intact a leg (42) to which the device is connected and supporting girders directly supporting the leg (42) support. 10

20. Method according to any of the claims 14-19, comprising positioning the vessel (50) in the vicinity of the device and providing one or more connecting cables (75) between the device and the vessel for transferring: control signals, electrical power and / or hydraulic fluids from the vessel to the device or vice versa.

A method according to any of claims 14-20, comprising: positioning a vessel above the device (10) comprising a hoisting device (87) and providing at least one hoisting cable (84) between the hoisting device and a connecting member (82 ) on the movable arm, performing an operation with the tool (60) mounted on the distal end of the movable arm, during which an additional hoisting force (F) is exerted by the vessel on the movable arm via the hoisting cable.