WO2024235817A1

WO2024235817A1 - Telescopic boom and crane, vessel and method comprising such a boom

Info

Publication number: WO2024235817A1
Application number: PCT/EP2024/062829
Authority: WO
Inventors: Jasper Hendrik Frank BAKKER
Original assignee: Itrec B.V.
Priority date: 2023-05-12
Filing date: 2024-05-08
Publication date: 2024-11-21
Also published as: NL2034817B1

Abstract

The invention relates to a telescopic crane boom (1). The boom comprises a main boom (10) and an extendable boom (20), and is provided with a stay mechanism (25) comprising a strut (30), a lower stay (40) and an upper stay (49). The upper stay comprises a first upper member (50) and a second upper member (60), that are each able to hinge between a passive position, for when the extendible boom is retracted, and an active position, for when the extendable boom is extended. In the passive position, the first upper stay member is positioned along the strut and the second upper stay member is positioned along the extendible boom. In the active position, the first and second upper stay members are connected to each other via connectors (61), and the upper stay forms a connection between the strut and the extendible boom structure.

Description

TELESCOPIC BOOM AND CRANE, VESSEL AND METHOD COMPRISING SUCH A BOOM

The invention relates to a telescopic crane boom.

In many fields of construction or maintenance, e.g. in the field off-shore wind turbines, there is a need to work at increasing heights. This means that cranes need to have to reach higher than previously. One way to do this is by increasing the length of the boom, however, this makes the transportation of the boom much more difficult. A second way to increase the height at which the crane can operate is by using a telescopic boom.

Telescopic crane booms are a type of boom that can be used for several lifting applications. A telescopic boom consists of a main boom section and one or more extendible boom sections that are telescopically received inside of the main boom section. The telescopic boom sections can telescope in and out of the main boom section, thus increasing the total height of the boom. This allows forthe boom to be transported in a compact configuration, with the telescopic boom sections retracted, while still enabling a high reach in a working configuration, with the telescopic boom sections extended. Thus, a relatively short boom that can be extended to the desired working height at the construction site.

Booms are usually equipped with stay mechanisms to provide stability and strength to a boom. Commonly the stay mechanism for telescopic boom consist of extendible cables or wires attached at the telescopic boom. See for example EP0637563. However, as the height and the loads that these booms need to lift increase it is becoming more difficult for these traditional stay mechanisms to provide adequate strength and stability while lifting at ever increasing heights. This is especially problematic for lifting operations where precision at great hight is necessary, e.g. the installation of wind turbine blades.

The present invention aims to provide a telescopic boom having enhanced stability and strength, e.g. in view of the increasing demands in the field of offshore wind turbines. A further object of the invention is to provide a stable and strong boom that is suitable for lifting loads to large heights, e.g. heights above 150 meter, for example for mounting or dismounting blades to large capacity wind turbines. It is another object of the invention to provide a boom that, once connected to the super structure of a crane, can be extended when raised, i.e. when the boom is pivoted upwards.

The present invention therefore provides an telescopic crane boom according to claim 1 .

A telescopic crane boom comprising: - a main boom, extending along a longitudinal axis between a base end and a top end, wherein the main boom is at the base end provided with a crane connector for connecting the boom to a crane super structure;

- an extendible boom, extending along a longitudinal axis between a base end and a top end, the extendible boom being telescopically received in the main boom so as to be movable along the longitudinal axis of the main boom between a retracted position and an extended position; and

- a stay mechanism, said stay mechanism comprising:

- a strut, extending between a boom end and a top end, wherein the strut is at the boom end connected to the top end of the main boom;

- a lower stay, extending between a boom end and a strut end, wherein the lower stay is at the boom end connected to the base end of the main boom and at the strut end is connected to the top end of the strut; and

- an upper stay, said upper stay comprising:

- a first upper stay member, extending between a strut end and a connector end, wherein first upper stay member is at the strut end hingeable connected to the top end of the strut, and is at the connector end provided with a connector; and

- a second upper stay member, extending between a boom end and a connector end, wherein the second upper stay member is at the boom end hingeable connected to the top end of the extendible boom, and is at the connector end provided with a connector; wherein the first upper stay member and the second upper stay member are each able to hinge between a passive position, for when the extendible boom is in the retracted position, and an active position, for when the extendable boom is in the extended position, wherein the first upper stay member, when in the passive position, is positioned along the strut and preferably is retained along the strut with strut mounted retaining members, wherein the second upper stay member, when in the passive position, is positioned along the extendible boom and preferably is retained along the extendible boom with extendible boom mounted retaining members, and wherein, when the extendible boom is in the extended position and the first upper stay member and the second upper stay member are each in the active position, the first upper stay member and the second upper stay member are connected to each other via the connector at their respective connector ends, and the upper stay forms a connection between the upper end of the strut and the top end of the extendible boom. Thus, with a stay mechanism according to the invention, the strut is mounted at or near a top end of the main boom, and the upper stay member is connected to a top end of the extendible boom, the extendible boom being the only boom section that is telescopically received in the main boom.

The stay mechanism of a boom according to the invention enhances the stability and strength of the boom compared to typical booms by providing a connection between the top end of the extendible boom in the extended position and the main boom. Due to the two upper stay members being able to connect to and disconnect from each other it is possible to use rigid members to form the upper stay, which provides considerably more stability and strength for the boom compared to the typical telescopic booms which use only cables or wires.

Furthermore, the stay mechanism of a boom according to the invention allows for a telescopic crane boom that, when extended, has a significant length. It is submitted that the stay mechanism is in particular suitable for long booms comprising only two telescopic parts. That is, wherein each of the two boom parts are of significant length, and the boom when extended has a length of over 150 meters.

Also, a stay mechanism according to the invention allows for the first and second upper stay member to be coupled by moving the extendible boom towards the extended position. This allows for the boom the be extended when it is raised, that is when it is pivoted upwards. Thus, the boom can in a retracted configuration be combined in a horizontal position with a crane super structure, subsequently be pivoted from the horizontal position into a raised position, and only then be extended. This allows for operating the crane on a small foot print, which is in particular beneficial on vessels and off shore structures.

In an embodiment, the main boom and/or the extendible boom is/are provided with a receiving space for receiving the second upper stay member when the second upper stay member is in the passive position and the extendible boom is in the retracted position. For example, in an embodiment, the main boom has a space for telescopically receiving the extendable boom, wherein said space has a height that is higher than the height of the extendable boom, such that there is a space between an upper surface of the extendable boom and the main boom for receiving the second upper stay member when the extendible boom is received within the main boom. In such an embodiment, at least part of the second stay member is located within the main boom when the extendable boom is in the retracted position.

Providing a receiving space in the main boom and/or in the extendable boom, allows for the second upper stay member to be in the passive position, in which it is positioned along the extendible boom, when the extendable boom is retracted in the main boom. Furthermore, providing the receiving space thus enables connecting the first upper stay member with the second upper stay member by moving the extendible boom from the retracted position towards the extended position. In an alternative embodiment, the second upper stay member, when in the passive position, is parallel to the extendable boom and is spaced from the extendable boom. In such an embodiment, there is a space between the extendable boom and the second upper stay member for receiving part of the main boom when the extendable boom is in the retracted position. In such an embodiment, the second upper stay member is located outside and along the main boom when the extendable boom is in the retracted position. In a further embodiment, the main boom is provided on the outside with a guide and/or support for respectively guiding the second upper stay member and supporting the second upper stay member when the extendible boom is moved towards, or is positioned in, the retracted position. Spacing the second upper stay member relative to the extendible boom, when the second upper stay member is in the passive position, provides a space between the second upper stay member and the extendible boom for receiving part of the main boom when the extendible boom is moved from the extended position into retracted position.

Thus, Spacing the second upper stay member relative to the extendible boom, allows for the second upper stay member to be in the passive position, in which it is positioned along the extendible boom, when the extendable boom is retracted in the main boom. Furthermore, spacing the second upper stay member relative to the main boom when in the passive position, thus enables connecting the first upper stay member with the second upper stay member by moving the extendible boom from the retracted towards the extended position.

In an embodiment, the strut has two legs, and provides a space between the legs for passing through the second upper stay member when the extendible boom is moved from the extended to the retracted position.

In such an embodiment, the second upper stay member passes below the strut, and is positioned between the extendable boom and the strut, when the extendible boom is moved from the extended position to the retracted position.

In a further embodiment, each of the two legs of the stay is connected to a mount provided on the main boom, wherein the mounts are provided on a left side and on a right side of the main boom, such that the second upper stay member passes between the mounts when the extendible boom is moved from the extended to the retracted position.

Providing a strut configured such that it straddles the second upper stay member allows for the second upper stay member to be in the passive position, in which it is positioned along the extendible boom, when the extendable boom is retracted in the main boom. Furthermore, providing such a strut enables connecting the first upper stay member with the second upper stay member by moving the extendible boom from the retracted towards the extended position.

In an embodiment wherein the second upper stay member is received within the main boom, when the extendable stay is retracted and the second upper stay member is in the passive position, the strut may be mounted on an outside surface of the main boom, and thus not necessarily needs to be mounted on opposite sides of the second upper stay member. In an embodiment, the second upper stay member is provided with a first upper stay member guiding structure, e.g. rails or a groove. The first upper stay member guiding structure is configured to engage and guide the connector of the first upper stay member and to limit lateral movement of the connector, while the extendible boom is moved from the retracted position into the extended position, to ensure that the connector of the first upper stay member connects with the connector of the second upper stay member.

As an alternative, or in addition, a first upper stay member guiding structure, e.g. rails or groove, is provided on the extendable boom at the rear side thereof, In such an embodiment, the guiding structure for engaging the connector of the first upper stay member is provided on the extendable boom, for example such that it extends alongside the second upper stay member when the latter is in the passive position. In an embodiment, a guide structure is provided on opposite sides, left and right, of the second upper stay member. In such an embodiment, the guide structure can form a cradle for receiving the second upper stay member when in the passive position. In such an embodiment, the main boom is preferably provided with a receiving space for receiving the second upper stay member and the first upper stay member guiding mechanism, when the second upper stay member is in the passive position. In a further embodiment, the first upper stay member guide mechanism is provided with retaining members for retaining second upper stay member when in the passive position.

In an embodiment, the boom comprises an extension mechanism for moving the extendible boom relative to the main boom. The extension mechanism comprises an extension winch, a winch driven extension cable, and an extension cable sheave system. The extension mechanism is configured to move the extendible boom from the retracted position into the extended position The extension mechanism therefore engages the extendible boom to provide an extension motion of the extendible boom. With the extension mechanism, the extendible boom can be moved from the retracted position into the extended position. In an embodiment the retraction motion of the extendible boom is caused by the mass of the extendible boom whilst the boom is upwardly oriented. In such an embodiment, the extension mechanism may be configured to lower the extendible boom into the retracted position instead of pulling it into that position. In an alternative embodiment, the extension mechanism is also configured to pull the extendable boom from the extended position into the retracted position.

In an embodiment, only the extendible boom is telescopically movable relative to the main boom. In an embodiment, the boom comprises an extendible boom locking mechanism, which locking mechanism preferably is positioned at the top end of the main boom. The locking mechanism is configured to lock the extendible boom relative to the main boom in at least the extended position and preferably in the retracted position. This allows for an extension mechanism to be unloaded while the boom is extended during use of the boom, and may be used to ensure that the extendible boom does not move out of the extended position when this is not preferred. In an embodiment, the boom comprises a boom head at the top end of the extendible boom, for supporting a main hoisting cable, the main hoisting cable extending between the boom head and a load connector supported by the main hoisting cable. In such an embodiment, a load that is hoisted by the crane is supported at the top end of the extendable boom. In an embodiment, this is also the location at which the second upper stay member is attached to the upper stay, the stay mechanism thus providing optimal support for the upper stay.

In an embodiment, the hoisting cable is looped multiple times between the boom head and the load connector. In such an embodiment, the boom head and the load connector are each provided with multiple sheeves for guiding the hoisting cable.

In a further embodiment, the boom comprises a main hoisting system. The main hoisting system comprises the boom head at the top end of the extendible boom, a main winch, a load connector, and an associated main hoisting cable, wherein the main hoisting cable extends between the main winch and the boom head.

The main hoisting system can be used to lift loads, e.g. wind turbine components using the boom. Providing the boom with a main hoisting system may facilitate and/or accelerate the installation of the boom on a crane superstructure because no mayor parts of the main hoisting system need to be mounted on the crane super structure when the boom is connected to the crane super structure.

In a further embodiment, the main winch of the main hoisting system is mounted on the main boom. In such an embodiment, the boom can be transported with the hoisting cable extending between boom and load connector. By providing the hoisting system in combination with the boom, the boom is more or less ready to be used once connected to a crane super structure because there is no need to install the hoisting cable by running it from a crane mounted main hoisting winch via the boom head to the load connector. Installation of the hoisting wire, which comprises running it over multiple sheeves, and may comprise the hoisting wire needing to be looped multiple times between the boom head and the load connector, and therefore may be a time consuming process. In an alternative embodiment, the main winch of the main hoisting system is configured to be mounted on the super structure of a crane, that is, is configured to be mounted on the super structure to which the boom is connected with the crane connector. Thus there is still no need to install the hoisting cable after the boom is connected with a crane super structure, only the hoisting winch needs to be connected to the crane super structure. Connecting the winch of the main hoisting system to a super structure may require less time than installation of a hoisting cable. The latter has to be installed over multiple sheeves and might have to be looped multiple times between the boom head and the load connector.

In an embodiment, the boom, preferably the extendible boom, is provided with a mount for temporarily connecting to a load connector, preferably to a load connector configured to be connected to the boom. Thus, the load connector may be mounted to the boom during transport of the boom, i.e. with the extendible boom in the retracted position. In an embodiment, the boom is provided with a boom mounted part of a luffing device, the boom mounted part of the luffing device preferably comprising a luffing wire for luffing the boom when connected to a crane superstructure. The boom mounted part of the luffing device is configured to cooperate with a crane mounted part of a luffing device to in combination form a luffing device for luffing the boom when the boom is connected to a crane supper structure.

Providing the boom with at least part of a luffing device may facilitate and/or accelerate the installation of the boom on a crane superstructure because only part of the luffing device has to be installed after the boom is mounted.

In a preferred embodiment, the boom mounted part of the luffing device comprises a luffing winch that is associated with the luffing wire, for when the boom is mounted to the super structure of a crane, paying in and paying out luffing wire to pivot the boom upwards and downwards. The luffing winch may be mounted on the boom, or may be configured to be mounted on the crane super structure.

In a preferred embodiment, the boom mounted part of the luffing device comprises a luffing crane block and a luffing boom block, wherein the luffing wire is looped multiple times between the luffing crane block and the luffing boom block. In such an embodiment, the luffing boom block is mounted to the boom, preferably to the main boom, and the luffing crane block is configured to be mounted to the super structure of the crane. Thus, when the boom is connected to the super structure of the crane, only the crane block has to be mounted to the super structure of the crane to install the luffing wire. Thus there is no need to install the luffing wire, i.e. to run the luffing wire multiple times between the crane block and the boom block after the boom is connected to the crane super structure. This may save time and allows for an comparatively quick and simple process for installation of the boom.

In an embodiment, the main boom and the extendible boom each have a length, and the length of the extendible boom is at least 80% of the length of the main boom, preferably at least 100% of the length of the main boom.

In a further embodiment, when the extendable boom is in the retracted position, at least 75%, preferably at least 90%, of the length of the extendible boom is received in the main boom.

In a further embodiment, when the extendable boom is in the extended position, at least 70%, preferably at least 80%, of the length of the extendible boom is outside the main boom.

In an embodiment, when the extendible boom is in the extended position, at least 80%, preferably at least 90%, most preferably 100%, of the length of the boom is provided by the main boom and the extendable boom, and preferably the part of the extendable boom outside the main boom forms at least 50% of the length of the boom.

In an embodiment, when the extendible boom is in the retracted position, the boom has a length in the range of 80-120m. For example, in a practical design, the boom has a length of more than 100 meters, e.g. of about 110 meters, when the extendible boom is in the retracted position. In an embodiment, when the extendible boom is in the extended position, the boom has a length in the range of at least 150-200m. For example, in a practical design, the boom has a length of more than 160 meters, e.g. of about 170 meters, when the extendible boom is in the extended position.

In an embodiment, the first upper stay member has a length equal to or smaller than a length of the strut. Preferably, the length of the first upper stay member is such that the connector of the first upper stay member is supported at a position for engaging the connector of the second upper stay member, when the second upper stay member is moved along the strut, for example below the strut or between two legs of the strut, when the extendible boom is moved from the retracted position into the extended position.

In an embodiment, the lower stay comprises one or more members, e.g. one or more linked steel bars. Composing a stay out of one or more members, such as steel bars, provides the stay with additional stiffness compared to a stay comprising only cables or chains.

In an embodiment, the first upper stay member is a single member, for example a steel bar or truss.

In an embodiment, the second upper stay member is a single member, for example a steel bar or truss.

In an embodiment, the first and second upper stay both comprise a single member. In such an embodiment, the first upper stay member and the second upper stay member, when in the active position, together form a straight connection between the upper end of the strut and the top end of the extendible boom. The use of a single member, e.g. a bar or truss, for the first upper stay member and for the second upper stay member provides a stiffer connection between the top end of the strut and the tip of the extendible boom, when the extendible boom is in the extended position. This increases the stability and strength of the crane compared to typically used non-rigid stays, e.g. stays consisting of wires.

It is submitted that, when the first and second upper stay member are in the active position, and together form a straight connection, then the first and second upper stay member are in line with each other.

In an embodiment, the main boom has a left boom leg and a right boom leg. Preferably, each boom leg is embodied as a hollow box structure, preferably a latticed hollow box structure, having a cross section defined by an inner side face and multiple further faces, wherein the inner side faces of the left boom leg and right boom leg face each other and are spaced from one another to form a clearance between the boom legs, and wherein the inner side faces have parallel rear and front chords, i.e. chords located relative to each other at the rear and front of the boom respectively. In an embodiment, the extendible boom is over at least a major portion of the length thereof, e.g. over at least 75% thereof, embodied as a hollow box structure, e.g. a rectangular cross section hollow box structure, preferably a latticed hollow box structure.

In a further embodiment, each boom leg is embodied as a hollow box structure, preferably a latticed hollow box structure, having a cross section defined by an inner side face and multiple further faces, wherein the inner side faces of the left boom leg and the right boom leg face each other and are spaced from one another to form a clearance between the boom legs, and wherein the inner side faces have parallel rear chords and front chords.

In an embodiment, the main boom comprises cross members interconnecting the left boom leg with the right boom leg, and wherein the cross members comprise rear trusses arranged in a plane between the rear chords of the right and left boom leg, and front trusses arranged in a plane between the front chords of the right and left boom leg, such that the distance between the rear and the front trusses is larger than the height of the extendible boom as to allow the extendible boom to be moveable between a retracted and an extended position in a longitudinal direction along the longitudinal axis of the main boom.

In an embodiment, the main boom comprises a left boom leg and a right boom leg, and the main boom comprises cross members, the cross members comprising rear trusses arranged at a rear side of the boom legs and front trusses arranged at a front side of the boom legs, that interconnect the left boom leg with the right boom leg. In this embodiment, the left and right leg, and the rear and front trusses, define between them a receiving space for receiving the extendible boom, when it is retracted inside the main boom, and that allows for the extendible boom to be moved between the retracted and the extended position, Preferably, the distance between the rear and front trusses is larger than the height of the extendible boom, and the receiving space is large enough for receiving both the extendible boom as the second upper boom member, the second upper boom member being positioned between the rear trusses and a rear side of the extendible boom when the extendible boom is in the retracted position and the second upper stay member is in the passive position.

In an embodiment, the main boom has cross members which interconnect the left boom leg with the right boom leg. Preferably, the cross members have rear trusses arranged in a plane between the rear chords of the boom legs and front trusses arranged in a plane between the front chords. To allow the extendible boom structure to be movable between a retracted and an extended position, the distance between the rear and front trusses is larger than the height of the extendible boom.

In an embodiment, the main boom has a guiding structure, e.g. rails, the guiding structure guiding the extendible boom when moved between the retracted and extended positions. Preferably, the extendible boom structure is also provided with a corresponding guiding structure that can engage with the guiding structure of the main boom. Preferably, the guiding structure of the main boom and/or the guiding structure of the extendible boom are/is equipped with rollers and/or wheels as to allow for easier movement of the extendable boom.

In an embodiment, the main boom has a guiding structure, e.g. rails, along the left and right boom leg. Preferably, the guiding structure extends along inner side faces of the left boom leg and the right boom leg. The extendible boom is slidably mounted on the guiding structure. The guiding structure making the movement between the retracted and extended positions of the extendible boom. Preferably, the extendible boom structure is also provided with a corresponding guiding structure that can engage with the guiding structure of the main boom. Preferably, the guiding structure of the main boom and/or the guiding structure of the extendible boom are/is equipped with rollers and/or wheels as to allow for easier movement of the extendible boom.

In an embodiment, a crane is provided with the telescopic crane boom according to this invention, such that it can be used for lifting loads in different construction and maintenance industries, e.g. offshore construction.

In a further embodiment, the crane comprises:

- a base structure;

- a revolving superstructure which is slewable about a slew axis relative to the base structure, and wherein the superstructure is provided with a boom connector;

- a luffing device for pivoting the boom up and down, the luffing device comprising a luffing winch and a variable length luffing system; wherein the boom is connected with the crane connector to the boom connector of the crane, and wherein the boom is pivotal up and down about a horizontal boom pivot axis which is perpendicular to the longitudinal axis of the boom.

In an embodiment, the luffing device comprises:

- two luffing winches on the superstructure; and

- two luffing cables, each driven by a respective luffing winch, said luffing cables extending spaced from one another, e.g. parallel to one another, from luffing cable sheave assemblies mounted on the superstructure, e.g. on a gantry thereof, to respective luffing cable connectors on the boom head or on the distal sections of the boom legs, which luffing cable connectors are spaced apart from another in a width direction of the boom, e.g. are arranged on opposite sides of the boom head seen in width direction.

In an embodiment, the crane is placed on an offshore vessel, for example a jack-up type vessel, or is a mobile crane configured to be temporarily mounted on an offshore structure, for example on a floating foundation of a wind turbine. In an embodiment, the second upper stay member, when in the passive position, is positioned along the extendible boom and preferably is retained along the extendible boom with extendible boom mounted retaining members. The retaining members may for example be embodied as clamping devices that engage part of an upper stay member and hold it or may be embodied as blocking devices, that block movement of the upper stay member once in the passive position. In an embodiment, the boom comprises one or more retainer winches and associated retainer wires for pulling the first and/or second upper stay member into the passive position. For example, the boom may comprise a second upper stay member retainer winches and an associated retainer wire. The retainer wire extends between the retainer winch and the second upper stay member, preferably to the connector end of the second upper stay member, for pulling the second upper stay member from the active position towards the extendible boom and into the passive position. The retainer wire preferably is guided via one or more sheeves.

In an embodiment, the boom comprises one or more coupling winches and associated coupling wires for pulling the connector end of the first upper stay member towards the connector end of the second upper stay member and to enable coupling of the connector of the first upper stay member with the connector of the second upper stay member. For example, a coupling wire may be guided via the connector end of the first upper stay member towards the connector end of the second upper stay member. In such an embodiment, the coupling wire can be used to puller the connector ends towards each other. For example, once the extendible boom is in the extended position, or while the extendible boom is moved towards the extended position, with the first and the second upper stay member in their passive position, the coupling wire can be used to pull the connector ends of the upper stay members towards each other, and thus to pull the upper stay members out of their passive position and into their active position.

In an embodiment, the connector of the of the first upper stay member and/or the connector of the second upper stay member comprises a hook element for engaging the other connector, for example a hook element of the other connector.

In an embodiment, the connector of the of the first upper stay member and/or the connector of the second upper stay member can be switched between a locked and an unlocked state, wherein in the unlocked state the respective connector allows for connecting and disconnecting the connector of the first upper stay member and the connector of the second upper stay member, and in the locked state the connector prevents disconnecting the connector of the first upper stay member and the connector of the second upper stay member.

For example, the connector may be provided with a latch and a drive for moving the latch between a stated for securing the connector of the first upper stay member and the connector of the second upper stay member in the connected condition. In an embodiment, a connector end is provided with a connector in the form of a hook, and the connector end of the other upper stay member is provided with a connector in the form of a pin, wherein the hook and the pin are adapted for cooperation, i.e. are adapted for pivotably coupling the connector end of the one upper stay member to the connector end of the other upper stay member. In a further embodiment, the hook type connector is provided with a locking device, for, when the upper stay members are pivotably coupled, locking the connection. In a preferred embodiment, the locking device is an automatic locking device, configured for establishing a locked coupling by moving the extendable boom from the retracted position towards the extended position.

According to the invention, the strut is at the boom end connected to the top end of the main boom. The strut, more in particular the boom end of the strut, can be located at the tip, or can be located more remote form the boom tip. Also, according to the invention, the second upper stay member is connected to the top end of the extendable boom. The second upper stay member, more in particular the boom end of the second upper stay member, can be located at the tip or can be located more remote from the tip of the extendable boom.

In an embodiment, the strut is at the boom end pivotable connected to the top end of the main boom, in a further embodiment, a support is provided, for example a support arm extending between the main boom and the strut, that holds the strut at an angle, preferably a 90 degree angle, to the main boom when the first upper stay member and the second upper stay member are not in their active positions, for example are in their passive positions.

The invention furthermore provides a crane configured to be provided with, preferably provided with, a boom according to the invention.

In a method according to the invention, a crane superstructure configured to be provided with a boom according to the invention and a boom according to the invention are transported separate from each other to a location, for example to an offshore wind turbine, and are combined only after arrival at the location, for example after the crane and the boom are transferred onto a wind turbine foundation.

The invention furthermore provides an offshore vessel, e.g. a jack-up type vessel, comprising a crane according to the invention.

The invention also relates to a method for extending a boom according to the present invention, wherein the boom preferably is part of a crane, e.g. is with the crane connector of the boom connected to a boom connector of a super structure of the crane. The method comprises the steps:

- moving the extendible boom, with the first upper stay member and the second upper stay member each in the passive position, along the longitudinal axis of the main boom out of the retracted position and towards the extended position, and thus moving the connector of the second upper stay member towards the connector of the first upper stay member, - connecting the connector of the first upper stay member with the connector of the second upper stay member, preferably by moving the extendible boom towards the extend position;

- moving the first upper stay member and the second upper stay member each out of the passive position and into the active position, preferably by moving the extendible boom towards the extend position, wherein preferably the first upper stay member and the second upper stay member, when in the active position together form a straight connection between the upper end of the strut and the top end of the extendible boom, and

- preferably, when the extendible boom is in the extended position, locking the extendible boom in the extended position to prevent movement of the extendible boom relative to the main boom

Preferably, while the extendible boom is moved out of the retracted position and into extended position, the connector of the second upper stay member engages and couples with the connector of the first upper stay member, and the second upper stay member subsequently pulls the first upper stay member such that the first and second upper stay members pivot about their connection points, at the top of the strut and the top of the extendible boom respectively, until the first and second upper stay members are in line with each other.

A further method according to the invention comprises the steps:

- prior to moving the first upper stay member and the second upper stay member each from the passive position towards the active position, releasing the first upper stay member from strut mounted retaining members, as to allow for rotational movement of the first upper stay relative to the strut, and releasing the second upper stay member from boom mounted retaining members, as to allow for rotational movement of the second upper stay member relative to the extendible boom.

The invention furthermore provides a method for hoisting an offshore wind turbine component, e.g. for installation and/or maintenance of an offshore wind turbine, wherein use is made of a crane according to the invention, i.e. a crane comprising a boom according to the invention, or a method according to the invention.

It is submitted that with the telescopic crane boom according to the invention, the a crane connector for connecting the boom to a crane super structure, preferably to a boom connector of the crane. In an embodiment, the crane connector and the boom connecter, once connected to each other, form a horizontal boom pivot axis for pivoting the boom up end down, preferably using a luffing system.

It is submitted that when the boom according to the invention is discussed, herein is referred to a bottom end of the boom as the end that is, or is to be, connected to a crane superstructure, and is referred to a top end of the boom as the end that, when the boom is in use, supports the hoisting cable, i.e. is provided with the crane block that supports the hoisting wire. Furthermore, when referred to the front side of the boom, that is the side that, when the boom is in use, faces a load supported by the crane, and the rear side of the boom is the side that, when the boom is in use, faces the crane super structure.

The invention also relates to a method for hoisting an offshore wind turbine component, e.g. for installation and/or maintenance of an offshore wind turbine, wherein use if made of a vessel and/or crane and/or boom and/or method as described herein.

The innovation will now be described with reference to the figures. In the figures:

Fig. 1 shows a side view of an exemplary embodiment of a telescopic boom according to the invention, the boom comprising an extendable boom telescopically received in a main boom;

Fig. 2 shows a top view of the boom of figure 1 , the boom having a left boom leg and a right boom leg;

Fig. 3 shows a side view of crane provided with the boom of figure 1 ;

Fig. 4 shows a cross sectional view of the boom, showing the main boom, and the extendible boom and the second upper stay member received therein.

Fig. 5a shows a side view of the boom of figure 1 with the extendible boom in a retracted position;

Fig. 5b shows a side view of the boom of figure 1 with the extendible boom in a first intermediate position;

Fig. 5c shows the boom of figure 1 with the extendible boom in a second intermediate position and with a first upper stay and a second upper stay in an intermediate position;

Fig. 5d shows the boom of figure 1 with the extendible boom in an extended position and with the first upper stay and the second upper stay in an active position;

Fig. 6a shows a side view in partial cross section of a boom according to the invention with a first upper stay member having a hook type connector in the passive position and a second upper stay member in the passive position; and Fig. 6b shows a side view of the boom of figure 6a with the first upper stay member locked to the second upper stay member via the hook type connector.

Figure 1 shows an exemplary embodiment of a telescopic crane boom 1 according to the invention, the boom 1 comprises a main boom 10, the main boom extends between a base end 12 and a top end 13. The main boom has a crane connector at the base end for connecting the boom to a crane super structure, e.g. to a revolving superstructure.

The telescopic boom further comprises an extendible boom 20, extending along a longitudinal axis between a base end and a top end. The extendible boom is telescopically received in the main boom so as to be movable along the longitudinal axis of the main boom between a retracted position and an extended position, figure 1 shows the extendible boom in the retracted position.

The telescopic boom has a stay mechanism 254, located at the rear side of the boom, i.e. the side of the boom that faces the super structure of the crane when the boom is in use. The stay mechanism comprises of a strut 30, a lower stay 40 and an upper stay 50. The strut has a boom end and a top end. The strut is connected at the boom end to the top end of the main boom 10.

The lower stay 40 has a boom end and a strut end. The lower stay is at the boom end connected to the base end of the main boom and at the strut end is connected to the top end of the strut. The lower stay forms a connection between the main boom 10 and the strut 30.

The upper stay comprises of two different parts, a first upper stay member 50 and a second upper stay member 60. The first and second upper stay members are both able to hinge between a passive position, when the extendible boom 20 is in the retracted position, and an active position when the extendible boom is in the extended position.

The first upper stay member 50 extends between a strut end 52 and a connecter end 53. The first upper stay member is at its strut end hingeable connected to the top end of the strut 30 and at the connecter end provided with a connector 51 . The first upper stay member, when in the passive position, is positioned along the strut. Figure 1 shows the first upper stay member in this passive position. Preferably the first upper stay member, when in the passive position, is retained with strut mounted retaining members.

The second upper stay member 60 extends between a boom end 62 and a connecter end 63. The second upper stay member is at its boom end hingeable connected to the top end of the extendible boom 20 and at the connecter end provided with a connector 61 . The second upper stay member, when in the passive position, is positioned along the extendible boom. Figure 1 shows the second upper stay member in this passive position. Preferably the second upper stay member, when in the passive position, is retained using extendible boom mounted retaining members.

The first and second upper stay members can be connected to each other via the connectors 51 , 61 at their respective connector ends, to form a connection between the upper end of the strut 30 and the top end of the extendible boom 20.

When the extendible boom is in the extended position, for example shown in figure 3, the first upper stay member and the second upper stay member are each in the active position. In this active position, the first upper stay member and the second upper stay member are connected to each other via the connector 51 , 61 at their respective connector ends. With the first and second upper stay members connected, the upper stay forms a connection between the upper end of the strut and the top end of the extendible boom, this provides the boom with increased stiffness, and thus allows for positioning objects at large heights.

In a shown embodiment, the main boom is provided with a receiving space 70 for receiving the second upper stay member 60 when the second upper stay member is in the passive position and the extendible boom 20 is in the retracted position. The second upper stay member is received inside of the main boom and along the extendible boom when the extendible boom is in the retracted position. Figure 4 shows a cross sectional view of the boom 1 , showing the main boom 1 , and the extendible boom 20 received therein.

In an alternative embodiment, in addition to the main boom, or as an alternative, the extendible boom is provided with a receiving space 70 for receiving the second upper stay member when the second upper stay member is in the passive position and the extendible boom 20 is in the retracted position. Thus, in such an embodiment, the main boom and the extendable boom together form a receiving space, or the extendable boom forms the receiving space, for receiving the second upper stay member when in the passive position, and enabling the extendible boom and the second upper stay member to be telescopically moved into the main boom.

In another embodiment, the second upper stay member is provided with a first upper stay member guiding structure, e.g. rails or groove. The guiding structure ensures that the connectors of the first and second upper stay member connect during the extending motion of the extendible boom. The guiding structure does this by engaging and guiding the connector 51 of the first upper stay member and by limiting the lateral movement of the connector.

In the embodiment shown, see figure 4, the strut 30 has two legs, preferably the two legs of the strut 250 are substantially parallel to each other. The two legs are arranged such that the legs provide a space between the legs 255 for passing through the second upper stay member. The space between the legs of the strut allow the second upper stay member to pass through said space when the extendible boom is moved from the retracted position to the extended position, and when the extendible boom is moved from the extended position to the retracted position.

In the shown example, each leg of the strut is connected to a mount which is provided on the main boom, preferably the mounts are provided on the top end of the main boom. One of the mounts is connected to the left boom leg 200 and other mount is connected to the right boom leg 210. The mounts are arranged such that the second upper stay member passes between the mounts when the extendible boom is moved from the retracted to the extended position and when the extendible boom is moved from the extended position to the retracted position.

In the example shown here, the first upper stay member 50 comprises two members, preferably the two members act as a single member.

In the illustrated embodiment, the first upper stay member 50 has a length equal to a length of the strut 30. Preferably, the first upper stay member has the same length as the strut to make connecting the connectors of the first and second upper stay member 60 easier. In the shown example, see figure 1 , the boom has an extension mechanism for the extendible boom 20. The extension mechanism comprises an extension winch 80, a winch driven extension cable 81 , and an extension cable sheave system. The extension mechanism engages the extendible boom to provide an extension motion of the extendible boom. The extendible boom can be moved from the retracted position into the extended position and vice versa.

The crane boom has an extendible boom locking mechanism 90 at the top end of the main boom 10. The locking mechanism is able to lock the extendible boom 20 in its extended position and preferably in its retracted position.

In the example shown here, the boom has a main hoisting system which can be used for hoisting loads. The main hoisting system comprises a boom head 100 placed at the top end of the extendible boom structure 20, a main winch 101 , preferably mounted on the main boom 10, and an associated main hoisting cable 102 which extends between the main winch and the boom head.

The main boom 10 and the extendible boom 20 have a length, the length of the extendible boom is at least 60% of the length of the main boom, preferably at least 110% of the length of the main boom.

The boom, when the extendible boom 20 is in the retracted position, has a length in the range of 80- 120m. For example, in a practical design, the boom has a length of more than 100 meters, e.g. of about 110 meters, when the extendible boom is in the retracted position.

The boom, when the extendible boom 20 is in the extended position, has a length in the range of at least 150-200m. For example, in a practical design, the boom has a length of more than 160 meters, e.g. of about 170 meters, when the extendible boom is in the extended position.

In the embodiment shown, the lower stay 40 comprises a bar. In an alternative embodiment, the lower stay can comprise, a rod, chain or a cable.

In the embodiment shown, the first and second upper stay 50, 60 both comprise a bar. The use of a bar for the first and second upper stay members provides a rigid connection between the tip of the extendible boom 20 in the extended position and the strut 30.

In the shown embodiment, the extendible boom 20 comprises a hollow box structure that has a rectangular cross section, preferably a latticed hollow box structure over a least a major part of the length of the extendible boom, e.g. over at least 75% of the length.

Figure 2 shows an alternative embodiment of the telescopic boom. The figure shows the rear side of the boom, i.e. the side of the boom facing the upper super structure 3 of the crane when in use. Again the extendible boom is shown in the retracted position. In this embodiment, the main boom has a left boom leg 200 and a right boom leg 210. The boom legs can run parallel to each other or taper toward each other.

In the shown embodiment, the boom legs have a latticed hollow box structure. The cross section of the boom legs is defined by an inner side face and multiple further faces. The inner side faces have parallel rear chords 220 and front chords. The inner side faces of the left boom leg face the side faces of the right boom leg. Furthermore, the side faces of the left boom leg are spaced apart from the side faces of the right boom leg, such that they form a clearance between them. In this clearance the extendible boom 20 is slidably mounted.

As illustrated, see also figure 4, the main boom 10 has a guiding structure 240, here shown as two sets of rails, along the inner side faces of the boom legs. The extendible boom 20 is slidably mounted on this guiding structure, the guiding structure allowing the extendible boom to move from the retracted to the extended position.

The main boom 10 has cross members 230 which interconnect the left and right boom leg. The cross members comprise rear trusses 231 in a plane between the rear chords of the right and left boom leg and front trusses 232 arranged in a plane between the front chords of the right and left boom leg. The rear and front trusses are spaced apart from each other at a distance larger than the height of the extendible boom 20. This allows the extendible boom to be mounted in between the left and right boom leg and it allows for the movement between the retracted and extended position of the extendible boom.

Figure 3, shows a crane with the telescopic boom of the present invention. Here the extendible boom 20 is in the extended position. The first and second upper stay member 50, 60 are in the active position, forming a straight connection between the top end of the extendible boom and the top end of the strut 30. The first and second upper stay member are connected to each other with their respective connectors.

In the embodiment shown, the crane comprises a base structure 300, a revolving superstructure 310 and a luffing device 320. The revolving superstructure is slewable about a slew axis relative to the base structure. The superstructure has a boom connector that is used to connect the boom to the superstructure, the boom has a corresponding crane connector that is connected to the boom connector of the superstructure, in the embodiment shown, the connection between crane connector and boom connector allows for the boom to be pivoted up and down about a boom pivot axis which is perpendicular to the longitudinal axis of the boom.

The luffing device 320 is used for pivoting the boom up and down. The luffing device comprise a luffing winch, a variable length luffing system 330 and a gantry 340. The variable length luffing system is connected between the gantry and the top end of the main boom 10. In an embodiment not shown, the crane with the telescopic boom is provided on an offshore vessel.

Figure 5a shows the first step of the method for moving the extendible boom 20 from its retracted position to its extended position. The first and second upper stay member 50, 60 are both in the passive position. The first upper stay member 50 is positioned along the length of the strut 30 and the second upper stay member 60 is positioned along the extendible boom 20. While moving the extendible boom to the extended position the connector 61 of the second upper stay member moves towards the connector 51 of the first upper stay member. The connector of the first upper stay member remains at the same position during the first step of the method.

Figure 5b show the moment that the first and second upper stay member 50, 60 are connected with the connectors 51 ,61 . In the embodiment shown, he connection is made by moving the extendible boom 20 towards the extended position. The figure shows the extendible boom in a partly extended position, that is in between the extended and the retracted position. The first and second upper stay members are in the passive position and now be moved towards the active position. Preferably, prior to moving the first upper stay and the second upper stay each from the passive position towards the active position, the first upper stay 50 is released from strut mounted retaining members, as to allow for rotational movement of the first upper stay relative to the strut 30, and the second upper stay member 60 is released from boom mounted retaining members, as to allow for rotational movement of the second upper stay member relative to the extendible boom 20.

Figure 5c shows the first and second upper stay member in a position between the passive position and the active position. The first upper stay member 50 is rotationally moved relative to the strut 30 and the second upper stay member 60 is rotationally moved relative to the extendible boom 20. The second upper stay member pulls the first upper stay member such that the first and second upper stay members pivot about a connection point between the first and second upper stay member. This rotational movement is caused by the extendible boom moving toward the extended position.

Figure 5d shows the first and second upper stay member 50,60 in the active position. The extendible boom 20 is in the extended position. The first and second upper stay member are aligned with each other and thus form a straight connection between the top of the strut 30 and the top end of the extendible boom. In the embodiment shown, when the extendible boom is in the extended position, the locking mechanism 90 is used to prevent the movement of the extendible boom relative to the main boom 10.

In an embodiment not shown, the telescopic boom is used for hoisting an offshore wind turbine component, e.g. for installation and/or maintenance of an offshore wind turbine.

According to the invention, the strut is at the boom end connected to the top end of the main boom.

The strut, more in particular the boom end of the strut, can be located at the tip, as for example is the case in the embodiment shown in figure 5a, or can be located more remote form the boom tip. Also, according to the invention, the second upper stay member is connected to the top end of the extendable boom. The second upper stay member, more in particular the boom end of the second upper stay member, can be located at the tip or can be located more remote from the tip of the extendable boom. The latter is the case in the embodiment shown in figure 5a.

Figure 6a shows a side view in partial cross section of a crane boom 401 according to the invention. The boom 401 comprises a main boom 405, a strut 406 mounted to a top end 407 of the main boom, a first upper stay member 402 and a second upper stay member 404.

In the embodiment shown, the first upper stay member 402 has a connector end 410 provided with a connector in the form of a hook 408, and the second upper stay member 404 has a connector end 411 provided with a connector in the form of a pin 409. The hook 408 and pin 409 are adapted for cooperation, i.e. are adapted for pivotably coupling the connector end of the first upper stay member to the connector end of the second upper stay member.

Furthermore, in the exemplary embodiment shown, the hook type connector 408 is provided with a locking device 412, for, when the first upper stay member and the second upper stay member are pivotably coupled, locking the connection. In the preferred embodiment shown, the locking device is an automatic locking device comprising a biased locking slide 413 and a biased activation arm 415.

The biased locking slide 413 is moveable supported by the first upper stay member. The locking slide can slide between a locking position, for securing the pin in the hook, and a release position, for allowing the pin to be engaged or disengaged by the hook. Figure 6a shows the locking slide in the release position and figure 6b shows the locking slide in the securing position. The locking slide is biased towards the securing position, in the exemplary embodiment shown by springs 414.

The biased activation arm 415 is moveable supported by the strut. The activation arm can be pivoted between an active position, in which it supports the biased locking slide in the release position, and a passive position in which it does not engage the biased locking slide, and the latter is in the release position. The biased activation arm is biased towards the release position by spring 416

When the extendable boom is in the retracted position, and the first upper stay member and the second upper stay member are in their respective passive position, the biased activation arm of the locking device engages the biased locking slide, and pushes the locking slide into a release position. This configuration is shown in figure 6a.

When the extendable boom is moved from the retracted position towards the extended position, the pin on the connector end of the second upper stay member is moved into the hook of the first upper stay member. The movement of the extendable boom subsequently pulls the first upper stay member and the hook away from the strut, thus activating the locking device. When the hook is pulled away from the strut, the biased activation arm pivots downwards and disengages the locking slide, and the locking slide is biased into a locking position. With the locking slide in the locking position , the pin is secure din the hook. Whis configuration is shown in figure 6b.

When the extendable boom is moved from the extended position towards the retracted position, the pin is initially secured in the hook. The movement of the extendable boom towards the retracted position moves the first upper stay member and the second upper stay member towards their respective passive positions. Moving the first upper stay member into its passive position makes that the strut mounted activation arm engages the first upper stay member mounted locking slide, and that the activation arm pushes the locking slide into the release position. The movement of the extendable boom towards the retracted position subsequently moves the pin out of the hook.

Thus, in the embodiment shown, the connector of the of the first upper stay member can be switched between a locked and an unlocked state, wherein in the unlocked state the respective connector allows for connecting and disconnecting the connector of the first upper stay member and the connector of the second upper stay member, and in the locked state the connector prevents disconnecting the connector of the first upper stay member and the connector of the second upper stay member.

It is noted that figure 6a and 6b show an exemplary embodiment of a connector that can be switched between a locked and an unlocked state. Alternative locking devices are also possible. For example, the connector of the first upper stay member may be provided with a latch and a drive for moving the latch between a state for securing the connector of the first upper stay member and the connector of the second upper stay member in the connected condition.

Reference signs

01 crane boom

02 crane

03 crane super structure

04 load connector supported by main hoisting cable

10 main boom

11 crane connector

12 base end main boom

13 top end main boom

14 longitudinal axis main boom

20 extendible boom

21 longitudinal axis extendible boom

22 base end extendible boom

23 top end extendible boom

25 stay mechanism

30 strut

31 boom end strut

32 top end strut

40 lower stay

41 boom end lower stray

42 strut end lower stay

49 upper stay

50 first upper stay member

51 connector at connector end first upper stay member

52 strut end first upper stay member

53 connector end upper stay member

60 second upper stay member

61 Connector at connector end second upper stay member

62 boom end second upper stay member

63 connector end second upper stay member

70 receiving space for receiving second upper stay member

80 extension winch

81 extension cable

90 extendible boom locking mechanism

100 head at top end extendible boom

101 main winch

102 main hoisting winch

200 Left boom leg

210 right boom leg 220 parallel rear chords inner side faces boom

230 cross members main boom

231 front trusses

232 rear trusses

240 guiding structure

250 two legs of strut

260 mount on main boom for leg of strut

300 base structure

310 revolving superstructure

320 luffing device for pivoting boom

330 variable length luffing system

340 gantry

401 crane boom

402 first upper stay member

403 hook type connector first upper stay member

404 second upper stay member

405 main boom

406 strut

407 top end main boom

408 hook type connector

409 pin

410 connector end first upper stay member

411 connector end second upper stay member

412 locking device hook type connector

413 biased locking slide

414 springs locking slide

415 biased activation arm

416 springs activation arm

Claims

C L A I M S

1 . Telescopic crane boom (1), the telescopic crane boom comprising:

- a main boom (10), extending along a longitudinal axis (14) between a base end (12) and a top end (13), wherein the main boom is at the base end provided with a crane connector (11) for connecting the boom to a crane super structure (3);

- an extendible boom (20), extending along a longitudinal axis (21) between a base end (22) and a top end (23), the extendible boom being telescopically received in the main boom (10) so as to be movable along the longitudinal axis (14) of the main boom between a retracted position and an extended position; and

- a stay mechanism (25), said stay mechanism comprising:

- a strut (30), extending between a boom end (31) and a top end (32), wherein the strut is at the boom end connected to the top end (13) of the main boom;

- a lower stay (40), extending between a boom end (41) and a strut end (42), wherein the lower stay is at the boom end connected to the base end (12) of the main boom (10) and at the strut end (42) is connected to the top end (32) of the strut (30); and

- an upper stay (49), said upper stay (49) comprising:

- a first upper stay member (50), extending between a strut end (52) and a connector end (53), wherein first upper stay member is at the strut end hingeable connected to the top end of the strut (32), and is at the connector end provided with a connector (51); and

- a second upper stay member (60), extending between a boom end (62) and a connector end (63), wherein the second upper stay member is at the boom end hingeable connected to the top end (23) of the extendible boom (20), and is at the connector end (53) provided with a connector (61); wherein the first upper stay member (50) and the second upper stay member (60) are each able to hinge between a passive position, for when the extendible boom (20) is in the retracted position, and an active position, for when the extendable boom (20) is in the extended position, wherein the first upper stay member (50), when in the passive position, is positioned along the strut (30) and preferably is retained along the strut with strut mounted retaining members, wherein the second upper stay member (60), when in the passive position, is positioned along the extendible boom (20) and preferably is retained along the extendible boom with extendible boom mounted retaining members, and wherein, when the extendible boom (20) is in the extended position and the first upper stay member (50) and the second upper stay member (60) are each in the active position, the first upper stay member (50) and the second upper stay member (60) are connected to each other via the connector (51 ,61) at their respective connector ends (53,63), and the upper stay (49) forms a connection between the top end (32) of the strut (30) and the top end (23) of the extendible boom (20).

2. Boom according to claim 1 , wherein the main boom (10) and/or the extendible boom (20) is/are provided with a receiving space (70) for receiving the second upper stay member (60) when the second upper stay member is in the passive position and the extendible boom (20) is in the retracted position.

3. Boom according to claim 1 or claim 2, wherein the strut (30) has two legs (250), and provides a space between the legs (255) for passing through the second upper stay member (60) when the extendible boom (20) is moved from the extended to the retracted position and when the extendible boom (20) is moved from the retracted position to the extended position.

4. Boom according to claim 3, wherein each leg of the strut (30) is connected to a mount (260) provided on the main boom (10), wherein the mounts (260) are provided on opposite sides of the main boom, such that the second upper stay member (60) passes between the mounts (260) when the extendible boom (20) is moved from the extended to the retracted position and when the extendible boom (20) is moved from the retracted position to the retracted.

5. Boom according to one or more of the preceding claims, wherein the second upper stay member (60) is provided with a first upper stay member guiding structure, e.g. rails or a groove, wherein the guiding structure is configured to engage and guide the connector (51) of the first upper stay member (50) and to limit lateral movement of the connector (51), while the extendible boom (20) is moved from the retracted position into the extended position, to ensure that the connector (51) of the first upper stay member (50) connects with the connector (61) of the second upper stay member (60).

6. Boom according to one or more of the preceding claims, wherein the boom (1) comprises an extension mechanism for moving the extendible boom (20) relative to the main boom (10), the extension mechanism comprising:

- an extension winch (80);

- a winch driven extension cable (81); and

- an extension cable sheave system, wherein the extension mechanism is configured to move the extendible boom (20) from the retracted position into the extended position.

7. Boom according to one or more of the preceding claims, wherein the boom (1) comprises an extendible boom locking mechanism (90), which locking mechanism preferably is positioned at the top end (13) of the main boom (10), and wherein the locking mechanism is configured to lock the extendible boom (20) relative to the main boom (10) in at least the extended position and preferably in the retracted position.

8. Boom according to one or more of the preceding claims, wherein the boom (1) comprises a boom head (100) at the top end of the extendible boom (20), for supporting a main hoisting cable, the main hoisting cable extending between the boom head and a load connector (4) supported by the main hoisting cable.

9. Boom according to claim 8, wherein the boom comprises a main hoisting system, the main hoisting system comprising the boom head (100) at the top end of the extendible boom, a main winch (101), a load connector and an associated main hoisting cable (102), wherein in the main hoisting cable extends between the main winch and load connector via the boom head.

10. Boom according to claim 9, wherein the main winch (101) of the main hoisting system is mounted on the main boom (10).

11 . Boom according to one or more of the preceding claims, wherein the boom (1) is provided with a boom mounted part of a luffing device, the boom mounted part of the luffing device preferably comprising a luffing wire for luffing the boom when connected to a crane superstructure, wherein the boom mounted part of the luffing device is configured to cooperate with a crane mounted part of a luffing device to in combination form a luffing device for luffing the boom when the boom is connected to a crane supper structure.

12. Boom according to one or more of the preceding claims, wherein the main boom (10) has a length and the extendible boom (20) has a length, and wherein the length of the extendible boom is at least 80% of the length of the main boom, preferably is at least 100% of the length of the main boom.

13. Boom according to claim 12, wherein, when the extendable boom (20) is in the retracted position, at least 75%, preferably at least 90%, of the length of the extendible boom (20) is received in the main boom (10).

14. Boom according to claim 12 or claim 13, wherein, when the extendable boom (20) is in the extended position, at least 70%, preferably at least 80%, of the length of the extendible boom (20) is outside the main boom (10).

15. Boom according to one or more of the preceding claims, wherein, when the extendible boom (20) is in the extended position, at least 80%, preferably at least 90%, most preferably 100%, of the length of the boom (1) is provided by the main boom (10) and the extendable boom (20), and preferably the part of the extendable boom (20) outside the main boom (10) forms at least 50% of the length of the boom (1).

16. Boom according to one or more of the preceding claims, wherein the boom (1), when the extendible boom (20) is in the retracted position, has a length in the range of 80-120 m

17. Boom according to one or more of the preceding claims, wherein the boom (1), when the extendible boom (20) is in the extended position, has a length of at least 150-200 m.

18. Boom according to one or more of the preceding claims, wherein the first upper stay member (50) has a length equal to or smaller than a length of the strut (30).

19. Boom according to one or more of the preceding claims, wherein the lower stay (40) comprises one or more members, e.g. one or more linked steel bars.

20. Boom according to one or more of the preceding claims, wherein the first upper stay member (50) is a single member, for example is a steel bar or a truss.

21 . Boom according to one or more of the preceding claims, wherein the second upper stay member (60) is a single member, for example is a steel bar or a truss.

22. Boom according to claim 20 and claim 21 , wherein the first upper stay member (50) and the second upper stay member (60), when in the active position, together form a straight connection between the upper end of the strut (30) and the top end (23) of the extendible boom (20).

23. Boom according to one or more of the preceding claims, wherein the main boom (10) comprises a left boom leg (200) and a right boom leg (210), and preferably wherein the extendible boom (20) is over at least a major portion of the length thereof, e.g. over at least 75% thereof, embodied as a hollow box structure, e.g. a rectangular cross section hollow box structure, preferably a latticed hollow box structure.

24. Boom according to claim 23, wherein each boom leg (200,210) is embodied as a hollow box structure, preferably a latticed hollow box structure, having a cross section defined by an inner side face and multiple further faces, wherein the inner side faces of the left boom leg (200) and the right boom leg (210) face each other and are spaced from one another to form a clearance between the left boom leg and the right boom leg, and wherein the inner side faces have parallel rear chords (220) and front chords.

25. Boom according to claim 19, wherein the main boom (10) comprises cross members (230) interconnecting the left boom leg (200) with the right boom leg (210), and wherein the cross members comprise rear trusses (232) arranged in a plane between the rear chords of the right and left boom leg (200), and front trusses (231) arranged in a plane between the front chords of the right and left boom leg (210), such that the distance between the rear and the front trusses is larger than the height of the extendible boom (20) as to allow the extendible boom to be moveable between a retracted and an extended position in a longitudinal direction along the longitudinal axis (14) of the main boom.

26. Boom according to claim 24 or claim 25, wherein the main boom (10) comprises a guiding structure (240), e.g. rails, along the left boom leg (200) and the right boom leg (210), preferably the guiding structure extends along inner side faces of the left boom leg and right boom leg, and wherein the extendible boom (20) is slidably mounted on the guiding structure, so as to be movable between the retracted position and the extended position.

27. Crane comprising a telescopic crane boom (1) according to one or more of the preceding claims.

28. Crane according to claim 27, wherein the crane comprises:

- a base structure (300);

- a revolving crane superstructure (310) which is slewable about a slew axis relative to the base structure, and the crane superstructure being provided with a boom connector; and

- a luffing device (320) for pivoting the boom (1) up and down, comprising a luffing winch and a variable length luffing system (330); wherein the boom (1) is connected with the crane connector (11) to the boom connector of the crane, and wherein the boom (1) is pivotal up and down about a horizontal boom pivot axis which is perpendicular to the longitudinal axis (14) of the boom (1).

29. Offshore vessel, e.g. a jack-up type vessel, comprising a crane according to claim 27 or claim 28.

30. Method for extending a boom according to one or more of the claims 1-26, wherein the boom (1) preferably is part of a crane, e.g. is with the crane connector (11) of the boom (1) connected to a boom connector of a super structure of the crane, wherein the method comprises the steps:

- moving the extendible boom (20), with the first upper stay member (50) and the second upper stay member (60) each in the passive position, along the longitudinal axis (14) of the main boom (10) out of the retracted position and towards the extended position, and thus moving the connector (61) of the second upper stay member (60) towards the connector (51) of the first upper stay member (50),

- connecting the connector (51) of the first upper stay member (50) with the connector (61) of the second upper stay member (60), preferably by moving the extendible boom (20) towards the extend position;

- moving the first upper stay member (50) and the second upper stay member (60) each out of the passive position and into the active position, preferably by moving the extendible boom (20) towards the extend position, wherein preferably the first upper stay member (50) and the second upper stay member (60), when in the active position, together form a straight connection between the top end (32) of the strut (30) and the top end (32) of the extendible boom (20); and

- preferably, when the extendible boom is in the extended position, locking the extendible boom in the extended position to prevent movement of the extendible boom relative to the main boom.

31 . Method according to claim 30, the method further comprising the steps:

- prior to moving the first upper stay member (50) and the second upper stay member (60) each from the passive position towards the active position, releasing the first upper stay member (50) from strut mounted retaining members, as to allow for rotational movement of the first upper stay member (50) relative to the strut (30), and releasing the second upper stay member (60) from boom mounted retaining members, as to allow for rotational movement of the second upper stay member (60) relative to the extendible boom (20).

32. Method for hoisting an offshore wind turbine component, e.g. for installation and/or maintenance of an offshore wind turbine, wherein use is made of a crane according to claim 27 or a method according to claim 30 or claim 31 .