ES2965249T3 - Low Construction Electric Luffing Jib Crane - Google Patents

Abstract

Crane (1) for unmanned platform (100) comprising a vertical main mast (2) connecting with the platform (100) and a cantilevered rotating crane arm (3) configured to perform lifting operations. The crane (1) further comprises a strut (4) having a fixed length, said strut (4) having a first end (4a) pivotally connected to said main mast (2) and a second end (4b) connected in a pivotal manner. sliding way to the crane. boom (3), said crane (1) further comprises a lifting device (20) and an electrically driven device (12, 53, 72) adapted to impose a force on the second end of the strut (4b) to move the second end (4b) back and forth along the longitudinal direction of the crane arm (3) between defined end positions (13a, 13b) arranged on the crane arm, the crane (1) further comprising a sliding mechanism ( 5) having a sliding path (13, 51, 71) formed along the longitudinal axis of the crane arm (3) and a yoke (7, 52, 72) pivotally connected to the second end of the strut ( 4b), said yoke (7, 52, 72) is adapted to move along the glide path (13, 51, 71). (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Low Construction Electric Luffing Jib Crane

Technical field

The invention relates to an electric crane for use on an unmanned wellhead platform. The invention relates in particular to an electric crane which at the same time has a reduced overall height.

Background of the technique

A typically unmanned offshore facility is a type of automated oil or gas platform designed to be operated primarily remotely, without the constant presence of personnel. A typical unmanned platform is shown in Figure 1 a.

These platforms are generally characterized by their small size. They often represent a compromise between providing the convenience of surface wellheads, which are easier to build and maintain, and avoiding the high operating cost of a full production platform.

Maintenance of unmanned platforms is typically performed from a larger nearby platform. Periodic visits may be made for routine maintenance and minor work in the wells, through the use of a service operation vessel to transport personnel and support operations. An example of such operations is wireline well intervention.

Although the platforms are small and most of the time unmanned, there is a need for equipment that meets safety standards when the platform is manned. The reduced space and height limitation during drilling periods also requires other solutions than normal platforms.

Due to the operation of these platforms, there are needs for new equipment that is adapted for these specific types of platforms.

There is a push in the industry to switch to all-electric cranes, as well as UWPs. There are many advantages to using electric cranes instead of other types of conventional cranes, such as hydraulic ones.

Electric cranes are, for example, environmentally friendly in that:

- No need for hydraulic oil.

- Low noise level.

- Energy savings due to low starting current and not having to continuously operate the crane machinery.

- The energy regenerated and the energy consumed are monitored.

- Power consumption/backup power control.

- Lower energy consumption (between 30 and 35 percent).

- Lower energy consumption could allow the choice of smaller models.

- Generator/transformer capacity.

Electric cranes are also very efficient in handling cargo, for example:

- Improved operational performance.

- Short response times.

- Maximum speed can be based on actual load.

- The exact position of the load is maintained even with the brake open.

- Short cycle times due to higher speeds.

- More precise operation ensuring easier load control and positioning.

There are also advantages in the repair and maintenance of electric cranes:

- Longer useful life of components.

- Easy maintenance.

- Lower maintenance costs.

- Less consumables.

- Improved bug finding.

- Easy access to data for problem resolution.

The improved charging cycle performance can be summarized as:

- Ramp time for each movement will be based on load; This will optimize the crane's response time.

- Response times will be shorter, giving operators an almost immediate reaction.

- Crane operation will be continuous with a maximum speed based solely on load and reach.

- The electric hoist can keep the load in an exact position even with the brake open, making it easier to handle the load.

When a jack-up rig is used to perform operations on the unmanned platform, the jack-up rig is arranged close to the platform and during drilling it will also have a cantilever drilling rig protruding from the jack-up rig. The cantilever drilling rig will then extend above the unmanned platform, making it unsuitable if you have tall obstacles, such as a tall crane, making movement of the cantilever rig difficult.

Figure 1a shows an example of an unmanned platform 100.

Figure 1 b shows the unmanned platform 100 with a jack-up platform 200 where the cantilever drilling rig 201 is shown. This cantilever drilling rig 201 could conflict with a crane having a tall tower.

An example of a prior art electric crane 150 is shown in Figure 2a. These cranes 150 comprise a main mast 151 and a lifting part 152, 152', 152" (shown in different positions) and have an extended tower 153. The lifting operation of the crane 150 is carried out by a rope or winch 154 connected between the tower 153 and the lifting portion 152. However, these types of cranes 150 are not suitable for use in conjunction with the jack-up platform 200 on unmanned platforms 100 as described above.

An example of a low construction crane 170 is shown in Figure 2b. These cranes 170 are also known as luffing jib cranes 170. The jack-up rig and cantilever drilling rig 201, as shown in Figure 1b, can easily be used in conjunction with the luffing crane 170 without any conflict between operations, as that the drilling cantilever can operate above the crane 170.

In the traditional low construction crane, as shown in Figure 2b, the luffing operation, that is, the movement of the crane arm in the vertical direction, is performed by a hydraulic mechanism.

However, functions such as the turning mechanism and lifting mechanism could be performed electrically.

Prior art low-build luffing cranes 170 have a crane arm 172 that is actuated by a hydraulic lifting mechanism 173 as shown in Figure 2b. The lifting mechanism or hydraulic cylinder 173 is arranged between a vertical main mast 171 which is connected to the platform 100 and to the bottom of the lifting portion or cantilever arm 172 of the crane to move the lifting portion 172 upward and down to perform the lifting operation.

None of the prior art lifting cranes 150, 170 shown in Figures 2a and 2b are of low construction and at the same time electric in lifting operations with all the advantages described above. GB 135704 A describes a crane comprising a vertical main mast and a cantilevered rotating crane arm configured to perform lifting operations, and further comprising a strut having a fixed length adapted to apply a pulling force on the arm. of the crane from above, where this strut has a first end pivotally connected to said main mast and a second end slidably connected to the crane arm. This crane further comprises a lifting system and an electrical drive device adapted to apply force on the second end of the strut to move the second end forward and backward along the longitudinal direction of the crane arm between defined end positions. arranged on the crane arm. This crane further comprises a sliding mechanism having a sliding channel formed along the longitudinal axis of the crane arm and a yoke pivotally connected to the second end of the strut, wherein said yoke is adapted to move along the channel. Sliding.

Therefore, an object of the invention is to provide a crane for an unmanned rig that has fully electric operations and at the same time is of low construction so that it can be installed and potentially used during the drilling phase with a jack-up rig.

Compendium of invention

The invention relates to a crane for an unmanned platform comprising a vertical main mast for connection to the platform and a cantilevered rotating crane arm configured to perform lifting operations. The invention is distinguished in that the crane further comprises a strut with a fixed length, adapted to apply a force on the crane arm from below, the strut having a first end pivotally connected to said main mast and a second end connected in a pivotal manner. sliding manner to the arm of the crane with a longitudinal sliding channel, said crane further comprising a lifting system having an electrical drive device adapted to apply a force on the second end of the strut to move the second end forward and backward along the longitudinal direction of the crane arm between the ends of the sliding channel.

The crane further comprises a sliding mechanism having a sliding channel shaped as an elongated opening disposed along the longitudinal axis of the crane arm and a pivotable yoke connected to the second end of the strut, said yoke being adapted to move within of the slip channel between the end points of the slip channel. The strut has a fixed length which results in a compressive force along the longitudinal axis of the strut when a force from the lifting system acts on the yoke.

An unclaimed embodiment relates to a crane for an unmanned platform comprising a vertical main mast and a cantilevered rotating lifting part, such as a crane arm, said lifting part being pivotally connected to the main mast. The invention is distinguished in that the crane further comprises a strut having a first pivoting end connected to the main mast and a second end arranged in a sliding manner within a longitudinal sliding channel located in the lifting part and which is allowed movement between defined extreme positions of the slip channel. Said crane further comprises a lifting system connected at one end to the second end of the strut. The lifting system has an electrical drive device adapted to apply a force on the second end for movement of the second end of the strut along the sliding channel to perform the lifting operation of the crane.

In another unclaimed embodiment of the crane, the strut is attached to the main mast below the lifting portion when the crane is installed on the platform.

In another embodiment of the invention, the sliding channel is formed as an elongated opening along the longitudinal axis of the crane arm, said yoke being adapted to move within the sliding channel (13).

In a further preferred embodiment of the invention, the lifting system comprises a cable or rope operably disposed between the second end of the strut and the electrical device, said electrical device being disposed on the arm of the crane.

In another embodiment of the invention, the lifting system further comprises a rigging system with a first return pulley fixedly attached to the crane arm and a second return pulley fixedly attached to the yoke, said cable or rope extending around the pulleys. first and second return valves before extending towards the electrical device.

In another embodiment of the invention, the electrical device and the first return pulley are arranged on opposite longitudinal sides of the sliding channel of the crane arm.

In a further embodiment of the invention, the electrical drive device is a winch.

In another embodiment of the invention, the lifting system comprises a screw mechanism having a frame portion fixedly attached to the crane arm and a helical grooved bar adapted to rotate about its longitudinal axis within the frame, said second end The strut is adapted to engage with the slots in the bar so that the second end of the strut is permitted to move into extreme positions in the frame portion when the bar is rotated.

In yet another embodiment of the invention, the lifting system comprises a rack and pinion mechanism, wherein the pinion is connected to the second end of the strut and is adapted to move along a rack provided in the arm of the crane. , said pinion is connected to an electrical device.

In another embodiment of the invention, the crane is provided with a working platform and handrail.

Brief description of the drawings

Figure 1a shows an example of an unmanned platform.

Figure 1b shows a jack-up platform with a cantilevered drilling rig extending above the unmanned platform.

Figure 2a shows a prior art electric crane with a high tower for performing lifting operations.

Figure 2b shows a prior art hydraulic luffing crane.

Figure 3 shows a perspective view of an electric low construction crane according to the present invention, with the lifting part or arm of the crane in the rest position.

Figure 4 shows a perspective view of an embodiment of the low construction crane lifting system according to the invention.

Figures 5-6 show the lifting system according to the embodiment shown in Figure 4, viewed from above.

Figures 7-8 show the lifting mechanism according to the embodiment of the invention shown in Figure 4, viewed from the side.

Figure 9 shows the low construction crane with the lifting portion or boom of the crane in the highest position.

Figures 10-13 show a detailed view of the lifting system embodiment in Figure 4 of the low construction crane viewed from different sides. The lifting part is in the highest position.

Figure 14 shows another embodiment of the lifting system with a screw system to move the second end of the strut along the lifting part or arm of the crane.

Figures 15-16 show another embodiment of the lifting mechanism with a rack and pinion system to move the second end of the strut along the lifting portion or arm of the crane.

Figure 17 shows a crane according to an unclaimed embodiment of an electric lifting system, seen in the rest position.

Figure 18 shows a crane according to the unclaimed embodiment of an electric lifting system, seen in a lifting position.

Figures 19-21 show a detail view of the crane according to the unclaimed embodiment of Figures 17-18.

Figure 22 shows a different arrangement of the electric drive according to the unclaimed embodiment shown in Figures 17-21.

Detailed description of the invention

The following describes a preferred embodiment of the present invention and some relevant prior techniques that are purely exemplary for a better understanding of the invention and not limiting.

Figure 3 shows an embodiment of a low construction electric crane 1 according to the present invention. The crane 1 comprises a vertical main mast 2 which is adapted to be fixedly attached to the platform 100. The crane 1 further comprises a lifting part or arm 3 of the rotating and cantilever crane. The crane arm 3 is pivotally connected at one end 4a to the main mast 2. The other end 3b of the crane arm 3 extends to a free end where lifting operations are performed. A strut 4 is pivotally connected at a first end 4a to the main mast 2 and is slidably or movably connected at a second end 4b to the arm 3 of the crane.

As shown in Figure 3, the first end 4a of the strut 4 could be connected between a pair of supports 6 that are connected to the main mast 2 of the crane 1.

The first end 4a of the strut is attached to the lower area of the main mast 2 as shown in Figure 1. The first end 4a of the strut is attached to the main mast 2 below the boom 3 of the crane.

Preferably the supports 6 could be arranged below the boom 3 of the crane so that the lifting operation is carried out from the bottom of the strut 4. There could be other connections between the main mast 2 and the strut 4 provided that the connection allows that the strut 4 rotates in relation to the main mast 2.

Strut 4 has a fixed length. Figure 4 also shows a sliding mechanism also called lifting mechanism 5 according to one embodiment of the invention. The sliding mechanism 5 will be described in more detail later. A lifting system 20 with a rigging system is also provided to reduce the forces necessary to perform the lifting operations. Furthermore, the lifting system 20 according to the embodiment of Figures 3-13 comprises an electrically powered device 12, such as a winch 12. A cable or rope 9 is arranged in operable contact with the electrically powered device 12, the rigging system and the second end 4b of the strut. A lifting system 20 according to the embodiment shown in Figures 3-13 comprises the electrically operated device 12, the rigging system 10, 11 and the cable or rope 9.

There could also be one or more work platforms 7a attached to the crane 1 for easy and safe access of personnel. The crane 1 could also be provided with handrails 7b as safety measures.

Figure 4 shows a perspective view of the lifting system 20 and sliding mechanism 5 shown in Figure 3.

The sliding mechanism 5 comprises a yoke 8 which is connected to the second end 4b of the strut. The second end 4b of the strut is pivotally connected to the yoke 8. The yoke 8 is arranged within an elliptical or similarly shaped opening 13 in the lifting arm. The opening is further referred to as slide channel 13 (see Figure 8).

The sliding channel 13 has end points 13a, 13b so that the yoke 8 with the strut 4 forms a rest position of the arm 3 of the crane when it is located at a first end point 13a. This position is shown in Figures 4-8. The yoke 8 with the strut 4 forms a position of maximum lifting of the arm 3 of the crane when it is located at a second end point 13b of the sliding channel 13. This position is illustrated in Figures 9-13.

The lifting system rigging system 20 further comprises a first return pulley 10 and a second return pulley 11. The cable or rope 9 extends around the first and second return pulleys 10, 11 an appropriate number of times. The first return pulley 10 is fixedly attached to the arm 3 of the crane. The first return pulley 10 could preferably be arranged near the end of the crane arm 3 connected to the main mast 2 of the crane 1.

The second return pulley 11 is connected to the yoke 8 or forms part of the yoke 8. The second return pulley 11 is adapted to slide together with the yoke 8 between the end positions 13a, 13b within the sliding channel 13. The cable or rope 9 is attached to the first return pulley 10 or the second return pulley 11 and extends several times around and between the return pulleys 10, 11 before the cable or rope 9 extends towards a winch 12. The The winch or electric drive device 12 is preferably arranged on the upper side of the arm 3 of the crane. The winch 12 is also preferably located further from the main mast 2 than the sliding channel 13, as especially shown in Figure 8. However, the position of the winch 12 is not limited to this position. The winch 12 could have other arrangements or positions on the crane arm 3.

The winch 12 and the first return pulley 10 are preferably arranged on opposite sides of the longitudinal direction of the sliding channel 13. It is important to have a small distance between the return pulley 10 because this makes it easier to wind the cable or rope 9 on the return pulleys 10, 11. Therefore, the forces that are required to move the second end 4b of the strut by the electrically driven device, for example the winch 12, could be reduced because a plurality of cable lengths or rope 9 extend between the first and second return pulleys 10, 11. The sections extend in parallel sections between the first and second return pulleys 10, 11.

The rigging system thus reduces the forces that the electrically operated winch device 12 has to exert on the second end 4b of the strut to move the second end 4b of the strut within the sliding channel 13.

Figures 5 and 6 show a detailed view of the lifting mechanism 5 according to the embodiment described in relation to Figure 4. These figures are seen in perspective.

The yoke 8 has two yoke parts 8a, 8b arranged on both sides of the second return pulley 11. Figure 5 shows the yoke parts 8a, 8b arranged in a sliding or movable manner within the arm 3 of the crane. In the figures, an upper structure 14 is arranged elevated with respect to the rest of the arm 3 of the crane. The upper structure 14 and parts of the crane arm 3 form a groove 15 on both sides of the second return pulley 11 in which the yoke parts 8a, 8b could rest and within which they could slide. The 15 slots could have different designs. Figure 5 shows a sectional cut between the upper structure 14 and the arm 3 of the crane. For illustrative purposes, Figure 6 shows a continuously designed upper structure 14 where the parts of the yoke 8 are completely covered within the arm 3 of the crane.

Figure 7 shows the crane of Figure 6, seen from the side. This figure illustrates that the sliding mechanism 5 is arranged inside the arm 3 of the crane.

In this case the sliding mechanism 5 is arranged in the upper part of the crane arm 3 for geometric reasons to obtain a better force reaction. The winch 12 is located on the upper surface of the crane arm 3.

Figure 8 shows a section within the crane arm where a portion of the crane arm has been removed. This figure shows in detail the lifting mechanism with the sliding channel 13, the first and second return pulleys 10, 11 and the yoke 8.

A part of the strut 4 extends inside the arm 3 of the crane. Therefore, on each longitudinal side of the crane arm 3 there is a downwardly extending crane arm cover 3c that covers a part of the strut 4. Between the crane arm covers 3c, a slot 3d is formed in which the strut 4 is adapted to move as shown in Figures 7-9.

Figure 9 shows a perspective view of the crane 1 where the crane arm 3 is in the highest position. This position of the crane 1 is shown in more detail in Figures 10-13, seen from different sides of the crane 1. The yoke 8 with the second return pulley 11 is located in the second end position 13b inside the sliding channel 13 near the first return pulley 10.

The operation of the low construction crane according to the embodiment shown in Figure 3-13 will be described in more detail below:

In the rest position of the crane 1, as shown in Figures 3-8, the crane arm 3 is arranged substantially horizontally. In this position the yoke 8 with the second end 4b of the strut is located in the first end position 13a of the sliding channel 13.

When the crane 1 must perform the lifting operation, the winch 12 will pull the cable 9 and collect it. This results in the first and second return pulleys 10, 11 that are in contact with the cable 9 being pulled closer to each other. Since the first return pulley is fixedly attached to the arm 3 of the crane, it will be the second return pulley that moves towards the first return pulley 10. The second return pulley 11 is connected to the yoke 8 and the second end 4b of the strut, which means that the second end 4b of the strut moves accordingly towards the first return pulley 10.

The strut 4 has a fixed length which results in a compressive force S along the longitudinal axis of the strut 4 when the force F from the lifting system 20 acts on the yoke 8. The lifting forces act along the longitudinal axis (shown as axis C in Figures 3 and 9) of the crane arm 3, along the sliding channel 13. The resultant force R is a lifting force directed upward on the upper side of the sliding channel 13. slip 13.

The lifting system 20 and the strut 4 apply the lifting force on the crane arm 3 until the second return pulley 10 is displaced to the second end point 13a of the sliding channel 13. The crane arm 3 is now in the highest position as shown in Figures 9-13.

The forces acting on crane arm 3 are illustrated in Figure 4 as S, F and R.

Figures 14, 15 and 16 show additional embodiments of the invention. Instead of the winch 12 and the rigging mechanism 5 to perform the lifting operation, as shown in Figure 3-13, the lifting operation could be performed in other ways, for example with a screw mechanism 50 or a lifting mechanism. rack and pinion 70, 80. These arrangements 50, 70, 80 are the sliding mechanism.

The figures show principle diagrams of the mechanism 50, 70, 80 that could replace the arrangement of winch 12 and cable 9 to move the end 4b of the strut between two extreme positions on the arm 3 of the crane.

The screw mechanism 50 could be arranged as a replacement for the rigging system as shown in Figures 3-13 or in addition to the rigging system, replacing only the winch 12 and cable 9 of the arrangement described above.

The screw mechanism 50 shown in Figure 14 has a frame structure 51 that could be arranged on the arm 3 of the crane. The screw mechanism 50 further comprises a rod 54 with helical grooves disposed within the frame 51. The rod 54 is adapted to rotate about its longitudinal axis by an electrical drive device 53.

A moving device, for example a beam 52, has an opening suitable for engaging with the bar 54 so that the beam 52 moves along the bar 54 when the bar 54 rotates. The strut end 4b could be connected to the beam 52 so that the strut end 4b moves accordingly when the bar 54 is rotated. The direction of rotation defines which direction the strut end 4b moves. The short sides 51a, 51b of the frame 51 define in this embodiment the end positions of the end 4b of the strut. The strut end 4b could also comprise an arrangement with an opening so that the strut end 4b can engage with the rod 54 and be moved along the rod 54 directly.

If the screw mechanism is arranged in connection with the rigging system 5 of Figures 3-13, the moving bar 54 could be coupled to a cable or rope 9 to move the strut 4.

Figures 15 and 16 show a typical rack and pinion arrangement 70, 80 that could be used in place of the winch 12, the rope or cable 9 and the rigging and rope mechanism 5 with the first and second return pulleys 10, 11.

In this embodiment, the end 4b of the strut is coupled to the rack or pinion and the crane arm is coupled to the other part of the rack or pinion. This results in the end 4b of the strut moving relative to the crane arm 3 along the longitudinal axis of the crane arm 3.

Figures 17-21 show embodiments that are not covered by the subject of the claims, of a low construction electric crane.

Parts that are similar in the embodiments have the same reference number in the drawing.

In this embodiment, the crane has a strut 111 having a first strut portion 112 and a second strut portion 113 that are telescopically movable relative to each other.

Figure 17 shows the crane 110 in the rest position with the second strut part 113 retracted into the first strut part 112.

Figure 18 shows the crane in the lifting position in which the second strut part 113 protrudes from the first strut part 112 so that the length of the strut 111 is extended.

An electrical drive mechanism 115, such as an electric motor, is attached to the strut 111 to effect movement of the second part 113 of the strut.

The embodiment with the extendable strut lifting system is shown in detail in Figures 19-21.

Figure 19 shows a detailed view of the crane 110 in the rest position with the second strut part 113 disposed within the first strut part 112. In the embodiment, the first strut portion 112 is pivotally attached to the vertical main mast at one end 112a. The connection could be, for example, through a support 6 that is fixedly attached to the main mast 2. This is similar to the embodiment described in Figures 3-13. The second strut 113 is pivotally connected at one end to the arm 3 of the crane. The connection to the arm 3 of the crane could be carried out, for example, through a support 114 that is firmly attached to the arm 3 of the crane. The bracket 114 is arranged at the bottom of the crane arm 3. The second part 113 of the strut and the pivotal connection to the crane arm 3 are shown in greater detail in Figures 20-20. In these figures, the crane 110 is in the same position as shown in Figure 18, where a portion of the second strut portion 113 extends from the first strut portion 112. Next, the crane arm 3 is raised in the vertical direction compared to the position shown in Figures 17 and 19.

The ends 112b, 113b opposite the pivotally connected ends 112a, 113a of the first and second parts 112, 113 of the strut are telescopically connected to each other. This results in the first and second parts 112, 113 of the strut being adapted to move relative to each other along the longitudinal axis. The longitudinal axis of the strut 111 is indicated by line L in Figure 20.

The second strut portion 113 could have threads 116 disposed along the outer surface of the second strut portion 113. The threads 116 could be arranged along all or partially around the outer surface of the second part 113 of the strut.

The first part 112 of the strut could have a disc 117 with an opening that is adapted to mesh with the threads 116 in the second part 113 of the strut.

The electric motor 115 is preferably connected to the first part 112 of the strut to rotate the disc 117 or the first part 112 of the strut. This is similar to a linear actuator.

The threads 116 are arranged helically so that when the disc 117 or the first part 112 of the strut is rotated about the longitudinal axis L of the strut 111, the second part 113 of the strut is forced outward or inward of the first part 112 of the strut resulting in the strut 111 extending or shortening depending on the direction of rotation.

Other embodiments of the strut 11 and the lifting system are also possible.

For example, the second strut part 113 with threads 116 could be pivotally attached to the vertical main mast 2. The first part 112 of the strut could then be pivotally connected to the arm 3 of the crane.

It is also possible for the electrically driven device 115' to be operably coupled to the second strut portion 113 to rotate the second portion 113 into or out of the first strut portion 112. This is illustrated in Figure 22.

The present invention has been described with reference to a preferred embodiment and some drawings solely for better understanding and it should be clear to those skilled in the art that the present invention includes all legitimate modifications within the scope of what has been described above in this document and what is claimed in the attached claims.

Claims (9)

1. A crane (1) for an unmanned platform (100) comprising a vertical main mast (2) for connection with the platform (100) and a rotating and cantilever crane arm (3) configured to perform lifting operations. lifting, where the crane (1) further comprises a strut (4) having a fixed length adapted to apply a force on the arm (3) of the crane from below, said strut (4) having a first end (4a) connected pivotally to said main mast (2) below the arm (3) of the crane and a second end (4b) connected in a sliding manner to the arm (3) of the crane, said crane (1) further comprises a system of lifting (20) and an electrically operated device (12) adapted to apply a force on the second end (4b) of the strut to move the second end (4b) forward and backward along the longitudinal direction of the arm (3 ) of the crane between defined end positions (13a, 13b) arranged on the arm of the crane, the crane (1) further comprising a sliding mechanism (5) having a sliding channel (13) formed along the longitudinal axis of the arm (3) of the crane and a yoke (8, 52, 72) pivotally connected to the second end (4b) of the strut, said yoke (8, 52, 72) being adapted to move along the channel of sliding (13), where the strut (4) has a fixed length that results in a compression force (S) along the longitudinal axis of the strut (4) when a force (F) from the lifting system (20) acts on the yoke (8). 2. The crane for an unmanned platform according to claim 1, wherein the sliding channel is formed as an elongated opening along the longitudinal axis of the crane arm, said yoke is adapted to move within the channel of sliding (13) between the end points (13a, 13b) of the sliding channel. 3. The crane according to any of claims 1-2, wherein the lifting system (20) comprises a cable or rope (9) operably arranged between the second end (4b) of the strut and the electrical device (12), said electrical device (12) is arranged on the arm (3) of the crane. 4. The crane according to claim 3, wherein the lifting system further comprises a rigging system with a first return pulley (10) attached to the arm (3) of the crane and a second return pulley (11). ) attached to the yoke (8), said cable or rope (9) extends around the first and second return pulleys (10, 11) before extending towards the electrical device (12). 5. The crane according to claim 4, wherein the electrical device (12) and the first return pulley (10) are arranged on opposite longitudinal sides of the sliding channel (13) of the arm (3) of the crane . 6. The crane according to any of claims 1 to 5, wherein the electrical drive device is a winch (12). 7. The crane according to any of claims 1 to 6, wherein the lifting system comprises a screw mechanism (50) having a frame part (51) attached to the arm (3) of the crane and a bar with helical grooves (52) adapted to rotate about its longitudinal axis within the frame, said second end (4b) of the strut is adapted to mesh with the grooves of the bar so that the second end (4b) of the strut is allowed the movement into extreme positions in the frame part (51) when the bar (52) is rotated. 8. The crane according to any of claims 1 to 7, wherein the lifting system comprises a rack and pinion mechanism, wherein the pinion (72) is connected to the second end of the strut (4) and is adapted to move along a rack (71) arranged in the arm (3) of the crane, said pinion is connected to an electrical device. 9. The crane according to any of claims 1 to 8, wherein the crane is provided with a work platform (7a) and handrail (7b).