WO2017082739A1

WO2017082739A1 - Knuckle-boom crane

Info

Publication number: WO2017082739A1
Application number: PCT/NO2016/050225
Authority: WO
Inventors: Baard Trondahl ALSAKER
Original assignee: MacGregor Norway AS
Current assignee: MacGregor Norway AS
Priority date: 2015-11-10
Filing date: 2016-11-10
Publication date: 2017-05-18
Anticipated expiration: 2018-05-10
Also published as: NO20151532A1

Abstract

There is described a knuckle-boom crane (1 ) for offshore hoisting operations comprising: - a knuckle-boom crane support means (2, 4); - a main boom (8) with a proximal end (12) and a distal end (16), the proximal end being rotatably connected to the knuckle-boom crane support means (2, 4) at a first rotation axis (10); - a main boom luffing means (13) for rotating the main boom (8) around the first rotation axis (10); - a knuckle-boom (14) rotatably connected to the distal end (16) of the main boom (8) at a second rotation axis (18), the knuckle-boom (14) comprising a first portion (20) including a first end (22) and a second portion (24) including a second end (26); - a load hoisting means (28), the load hoisting means (28) including an elongated hoisting member (30) and an elongated hoisting member drive means (32), a first end (34) of the elongated hoisting member (30) being connected to the elongated hoisting member drive means (32) and a second end (36) of the elongated hoist member (30) being connectable to a load suspension member; and - a knuckle-boom rotation means (42) for rotating the knuckle-boom (14) around said second rota¬ tion axis (18), wherein - the second rotation axis (18) is provided between the first and second portions (20, 24) of the knuckle-boom (14); and - the elongated hoisting member (30) is adapted to run over both said first portion (20) and said second portion (24) of the knuckle-boom (14) so that a force acting on the knuckle-boom from the elongated hoisting member creates a positive moment on the second portion (24) of the knuckle- boom and a negative moment on the first portion (20) of the knuckle-boom (14),wherein the knuckle-boom rotation means (42) comprises an elongated tensioning member (44) and an elongated tensioning member drive means (46), a first end of the elongated tensioning member (44) being connected to the elongated tensioning member drive means (46) and a second end of the elongated tensioning member being directly or indirectly connected to the knuckle-boom (14) at said first portion (20) thereof, and wherein the elongated tensioning member drive means (46) is connected to the main boom (8).

Description

KNUCKLE-BOOM CRANE

The invention relates to a knuckle-boom crane. In particular the invention relates to a knuckle- boom crane for offshore hoisting operations, wherein the knuckle-boom crane comprises a knuckle-boom crane support means; a main boom with a proximal end and a distal end, the proximal end being rotatably connected to the knuckle-boom crane support means at a first rotation axis; a main boom luffing means for rotating the main boom around the first rotation axis; a knuckle-boom rotatably connected to the distal end of the main boom at a second rotation axis, the knuckle-boom comprising a first portion including a first end and a second portion including a second end; a load hoisting means, the load hoisting means including an elongated hoisting member and an elongated hoisting member drive means, a first end of the elongated hoisting member being connected to the elongated hoisting member drive means and a second end of the elongated hoist member being connectable to a load suspension member; and a knuckle-boom rotation means for rotating the knuckle-boom around said second rotation axis.

Knuckle-boom cranes are known to be particularly useful in offshore environments, both because they occupy little deck space and because of their low centre of gravity compared to other cranes known to be used offshore. On a knuckle-boom crane, the main boom is hinged at the middle, thus creating a knuckle-boom. The luffing motion of both the main boom and the knuckle-boom is usually controlled with hydraulic cylinders. This way, movements of the load can be limited as the boom tip can be kept at a limited height above deck. This feature makes the crane both safe and efficient. The ability to knuckle in combination with the vessel's movement due to environmental conditions, imply that loads imposed to the crane structure will vary both in magnitude and direction.

Offshore lifting cranes, including knuckle-boom cranes, are getting increasingly large in order to satisfy the need to lift continually heavier weights, often in increasingly deep waters. Larger and heavier cranes require larger and heavier hydraulic cylinders to operate the booms, and larger and more powerful cylinders require more reinforcement at connection points. Heavy cranes and cylinders significantly contribute to huge fuel consumption in transit and also have high energy consumption in use. Further, heavy cranes put limitations to the maximum allowable payload, and also reduce the stability of the offshore structure, such as a vessel or platform, on which the crane is placed. Knuckle-boom cranes are typically used on a number of different Offshore Support Vessels (OSVs), such as Construction Support Vessels (CSVs), Platform Support Vessels (PSVs), and Anchor Handling Tug Supplies (AHTS) used in the offshore service and support industry, though the invention is not limited to be used on any of the mentioned kinds of vessel.

Further disadvantages of the prior art knuckle-boom cranes are the risk of spilling of oil from hydraulic cylinders and relatively complicated and costly installation processes. Finally, current state of the art hydraulic cylinders have a limited lifting capacity, and a further upscaling of knuckle-boom cranes are potentially limited by the obtainable capacity of the hydraulic cylinders.

Patent application WO 2014/014343 A1 discloses a knuckle-boom crane for marine applications, wherein hydraulic cylinders have been replaced by winch-operated wires/cables. The luffing motion of the knuckle-boom by means of a wire is enabled by letting the wire run from the crane housing to the knuckle-boom through a curved extension guide extending over the crane itself so that the wire connects the knuckle-boom from above. The knuckle-boom may as such be actively lifted by means pulling in wire on the winch, while lowering of the knuckle-boom may be initiated by letting out wire from the winch and letting the knuckle-boom be lowered by means of gravity.

The knuckle-boom crane disclosed in the patent application mentioned above solves some of the disadvantages of the prior art related to hydraulic actuators; limited possibilities for upscaling and potential spilling of oil, while other disadvantages, such as, increasingly heavy cranes and high energy consumption remain unsolved.

It is thus an object of the present invention to provide a knuckle-boom crane that is adapted for upscaling to heavier loads while at the same time reducing crane weight and energy consumption. The invention has for its general object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advanta- geous embodiments of the invention.

In a first aspect the invention relates to a knuckle-boom crane for offshore hoisting operations comprising:

- a knuckle-boom crane support means, such as a pedestal and/or a crane king;

- a main boom with a proximal end and a distal end, the proximal end being rotatably connected to the knuckle-boom crane support means at a first rotation axis;

- a main boom luffing means for rotating the main boom around the first rotation axis;

- a knuckle-boom rotatably connected to the distal end of the main boom at a second rotation axis, the knuckle-boom comprising a first portion including a first end and a second portion including a second end; - a load hoisting means, the load hoisting means including an elongated hoisting member, and an elongated hoisting member drive means, typically a winch, a first end of the elongated hoisting member being connected to the elongated hoisting member drive means and a second end of the elongated hoisting member being connectable to a load suspension member; and

- a knuckle-boom rotation means for rotating the knuckle-boom around said second rotation axis, wherein

- the second rotation axis is provided between the first and second portions of the knuckle-boom; and

- the elongated hoisting member is adapted to run over both said first portion and said second por- tion of the knuckle-boom so that a force acting on the knuckle-boom from the elongated hoisting member creates a positive moment on the second portion of the knuckle-boom and a negative moment on the first portion of the knuckle-boom, wherein the knuckle-boom rotation means comprises an elongated tensioning member and an elongated tensioning member drive means, a first end of the elongated tensioning member being connected to the elongated tensioning member drive means and a second end of the elongated tensioning member being

directly or indirectly connected to the knuckle-boom at said first portion thereof and wherein the elongated tensioning member drive means s connected to the main boom.

In the following the elongated hoisting member will be exemplified by a wire rope which may comprise steel and/or fibres, though the elongated hosting member may also be provided as any other suitable hoisting cable, including as a chain. The load suspension member typically comprises a hook to which the load may be connected.

In a knuckle-boom crane according to the invention, the forces acting from the elongated hoisting member on the knuckle-boom and contributing to a negative moment around the second rotation axis, may at least partially balance out a positive moment created by the weight of any load sus- pended from the knuckle-boom crane. This implies that the knuckle-boom rotation means only needs to lift a part of the weight of the knuckle-boom and any load suspended therefrom. As such, the weight of the knuckle-boom rotation means may be significantly reduced due to the lowered requirements for lifting capacity. The reduced requirement for lifting capacity may also lead to a reduced need for reinforcement in the booms, in particular at load concentration points, such as at any remaining cylinder connection.

The second end of the elongated tensioning member may be connected, directly or indirectly, to the knuckle-boom at said first portion thereof. Indirectly meaning that the elongated tensioning member may run over a sheave connected to said first portion. As such, a winch and a wire rope or other elongated hoisting member may at least partially be controlling the luffing motion of the knuckle-boom, i.e. its rotation around the second rotation axis. As such there may be no need to install cylinders to control the luffing motion of the knuckle-boom, which further may reduce the need for reinforcement of the main boom. It should be noted that it is the fact that the second rotation axis is provided between the first and second ends of the knuckle-boom that makes its luffing motion controllable by means of a winch and wire rope. In a conventional knuckle-boom crane, what has here been defined as the second rotation axis is provided at one of the ends (first end) of the knuckle-boom, hence a winch could not be used to lift the knuckle-boom without being connected to the knuckle-boom from above, as in WO 2014/014343 A1. In contrast, in a knuckle-boom crane according to the present invention, the load-carrying part (second portion) of the knuckle- boom may be lifted by pulling down the opposite portion of the knuckle-boom.

However, in addition said knuckle-boom rotation means may comprise one or more hydraulic or electric cylinders connected between the knuckle-boom and the main boom as in a conventional knuckle-boom crane. The mentioned balancing of the knuckle-boom by means of forces acting from the wire rope would also reduce the requirements for cylinder capacity, implying that the crane could be provided with smaller, lighter and less energy-consuming cylinders for controlling the luffing motion of the knuckle-boom.

In one embodiment, said knuckle-boom rotations means may comprise two elongated tensioning member drive means per elongated tensioning member. The elongated tensioning member may thus be spooled from one drive means, typically a winch, over to the other, and thereby significantly simplifying a process of replenishing or replacing such an elongated tensioning member, typically as it becomes worn. Normally, a knuckle-boom crane according to this embodiment of the invention would comprise two elongated tensioning wire ropes and two pairs of tensioning winches, one on each side of the knuckle-boom crane. The elongated tensioning member would in such an embod- iment not be directly connected to the first portion of the knuckle-boom, but rather be reeved over a sheave connected to the first portion of the knuckle-boom, from the first drive means to the second drive means.

By connecting the drive means for the elongated tensioning member directly to the main boom, a simple and compact design may be realized as the elongated tensioning member need not be run all the way from the pedestal or another part of the support structure of the crane. As mentioned above, two elongated tensioning members and two (pairs of) elongated tensioning member drive means may be provided, one on each side of the main boom.

In one embodiment, the elongated tensioning member may be directly or indirectly connected to the knuckle-boom, towards the first end thereof, thus contributing to what has been defined as a negative overturning moment about the rotation axis of the knuckle-boom by pulling down the first portion of the knuckle-boom. By indirectly is meant running over a sheave connected towards the first end as described above. This will typically be the case when the elongated tensioning member runs from a first drive means to a second drive means. Instead of lifting the whole weight of the knuckle-boom and any load in order to control the luffing motion of the knuckle-boom, the first part of the knuckle-boom is pulled down towards the main boom which results in a simultaneous lifting of the second part. However, since the forces acting from the elongated hoisting member on the knuckle-boom at least partially balances out the forces acting on the knuckle-boom, the elongated tension member only needs to pull in the first portion of the knuckle-boom by a limited force, equal to the unbalanced weight of the knuckle-boom and load, in order to lift the second portion of the knuckle-boom.

In one embodiment the knuckle-boom crane may additionally be provided with a knuckle-boom folding means, which may facilitate the folding of the knuckle-boom towards the main boom, which may be convenient for making and idle crane more compact and with a lowered center of gravity. The folding means may comprise an elongated folding member and an elongated folding member drive means connected to the main boom, the elongated folding member having a first end connected to the elongated folding member drive means and a second end connected to the knuckle- boom at the second portion thereof, so as to be able to pull in the knuckle-boom towards the lower side of the main boom, i.e. to fold the knuckle-boom. In an alternative embodiment, the folding means may comprise a hydraulic or electric cylinder adapted to retract the knuckle-boom towards the main boom. In alternative embodiments, the folding means may comprise a cylinder acting to lift/push the first portion of the knuckle-boom in a clock-wise direction away from the main boom, or a cylinder adapted to pull in the second portion of the knuckle-boom towards the main boom, where both the alternative versions also would fold the knuckle-boom towards the main boom.

In one embodiment the knuckle-boom rotation means may comprise a motor, such as an electric or hydraulic motor, provided at/around the second rotation axis and adapted to at least partially contribute in rotating said knuckle-boom around said second rotation axis. The motor may be used in addition to another knuckle-boom rotation means, or it may even operate as the sole knuckle-boom rotation means in cases where the weight of the load and knuckle-boom is substantially balanced out by the force acting from the elongated hoisting member. In one specific embodiment, the motor may be a permanent magnet motor.

In one embodiment the second rotation axis, i.e. the axis around which the knuckle-boom is rotata- ble relative to the main boom, may be provided substantially mid-way between the first and second ends of the knuckle-boom, where mid-way is defined in terms of distance. Alternatively said second rotation axis may be provided substantially at the centre of gravity of the knuckle-boom. This entails that there may be a significant moment arm of the knuckle-boom contributing to the negative moment on the first portion of the knuckle-boom, depending on the distance from the second rotation axis to where the forces from the wire rope act on the first portion of the knuckle-boom. In alternative embodiments said second rotation axis may be provided offset from mid-way in the direction of the first or second end, such as between 20-80 % of the length of the knuckle-boom, wherein 0 represents the first end and 100% represents the second end, preferably between 30-70 %, more preferably between 40-60 % and most preferably, as mentioned above substantially mid-way be- tween the ends, i.e. substantially at 50 %.

In one embodiment the knuckle-boom, at the first portion thereof, may be provided with a sheave over which the elongated hoisting member runs, the sheave thus being the point of attack of the forces acting from the wire rope on the first portion of the knuckle-boom. The further away the sheave is provided from the second rotation axis, the greater the moment. In one specific embodiment the sheave may be movably/slidably connected to the knuckle-boom so that the moment arm between the second rotation axis and the forces acting from the elongated hoisting member may be varied and thus potentially tailored and optimized for different loads.

There is also described a vessel comprising a knuckle-boom crane according to the first aspect of the invention. The vessel may be of a kind typically used in the offshore service and support industry, such as a number of different Offshore Support Vessels (OSVs), including Construction Support Vessels (CSVs), Platform Support Vessels (PSVs), and Anchor Handling Tug Supplies (AHTS), though the vessel is not limited to any of the mentioned types.

It should be noted that a knuckle-boom crane according to the present invention may be provided with means for measuring tension, moment, weight etc. and also a control unit adapted to receive measured data from said measuring means for control and potentially regulation of the crane, as will be understood by a person skilled in the art.

In a second aspect, the invention relates to a method for operating a knuckle-boom crane according to the first aspect of the invention, wherein the method comprises the steps of:

- operating the elongated hoisting member drive means so as to regulate the moment about the second rotation axis; and

- regulating the moment about the second rotation axis so as to maintain a positive moment around the second rotation axis independent of the load suspended from the second end of the elongated hoisting member.

In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein:

Fig. 1 shows, in a side view, a first knuckle-boom crane according to the present invention in a first position of use;

Fig. 2 shows, in a similar side view, a second knuckle-boom crane according to the present invention in a second position of use; and

Fig. 3 shows, in a schematic overview, a vessel comprising a knuckle-boom crane according to the invention.

In the following, the reference numeral 1 will refer to a knuckle-boom crane according to the present invention. In the figures, which are shown simplified and schematically, identical reference numerals will refer to identical or similar features. The various features in the figures are not necessarily drawn to scale. Figure 1 shows a knuckle-boom crane according to the present invention in a first position of use. The knuckle-boom crane 1 comprises a support means in the form of a pedestal 2 and a crane housing 4, the crane housing 4 being slewingly connected to the pedestal 2 by means of a not shown slew bearing, while the pedestal 2 itself is placed on deck of a vessel 6, as schematically indicated in Figure 3. A main boom 8 is rotatably connected to the crane housing 4 around a first rotation axis 10 at a proximal end 12 thereof. A main boom luffing means 13, here shown as a pair of cylinders, enables a luffing motion of the main boom 8 by rotating it around said first rotation axis 10. The cylinders 13 may be of hydraulic or electric type. A knuckle-boom 14 is connected to a distal end 16 of the main boom 8 at a second rotation axis 18. The second rotation axis 18 defines a first portion 20 including a first end 22 of the knuckle-boom 14 on one side of the second rotation axis 18 in a plane perpendicular to the second rotation axis 18 and a second portion 24 including a second end 26 of the knuckle-boom 14 on the other side of the second rotation axis 18 in said plane perpendicular to the second rotation axis 18, The knuckle-boom crane 1 further comprises a load hoisting means 28 comprising an elongated hoisting member 30 and an elongated hoisting member drive means 32, in the embodiment shown as a hoisting wire rope and a hoisting winch respectively. The hoisting wire rope 30 has a first end 34 connected to the hoisting winch 32 and a second end 36 connectable to a not shown load via a not shown load suspension member. The hoisting winch 32 is connected to an upper portion of the crane housing 4 at a side of the crane housing 4 facing away from the knuckle-boom 14. The hoisting wire rope 30 extends from the hoisting winch 32 along with the main boom 8, into a first guiding sheave 33 rotatably connected to the first portion 20 of the knuckle-boom 14, along with the knuckle-boom 14, over a second guiding sheave 35 at the second portion 24 of the knuckle-boom 16 and exiting the knuckle-boom 14, further between third and fourth guiding sheaves 38, 40 at the second end 26 of the knuckle-boom 14 and ending up at the not shown load suspension member. In order to control the luffing motion of the knuckle-boom 14, the knuckle-boom crane 1 is provided with a knuckle-boom rotation means 42. In the shown embodiment, the knuckle-boom rotation means 42 includes an elongated tensioning member 44 in the form of a tensioning wire rope, or rather two tensioning wire ropes, one each side of the main boom 8, and an elongated tensioning member drive means 46 in the form of a pair of small winches, or rather two pairs of small winches, one pair on each side of the main boom 8, with only one pair of winches 46 visible in the figure. Each tensioning wire rope 44 runs from one winch 46 connected to the main boom over a first small guiding sheave 48 on the main boom 8, up to a second small guiding sheave 50 at first end 22 of the knuckle-boom 14, back to a third small guiding sheave 52 on the main boom and to a second small winch 46 also connected to the main boom 8. The use of two winches 46 for each tensioning wire rope 44 enables the use of "cut-and- slip" to simplify replacement or replenishing of worn wire rope. By pulling in the tensioning wire ropes 44, what has been defined as a negative moment S2 is created about the second rotation axis 18 from the tensioning wire ropes 44 pulling on the small guiding sheaves 50 at the first end 22 of the knuckle-boom 14, attempting to rotate the knuckle-boom 14 in a counter clockwise direction around the second rotation axis 18. The required moment for rotating the knuckle-boom 14 around the second rotation axis 18 may be significant, in particular if a load is connected to the hoisting wire rope 30. The load will contribute to a positive moment S2 around the second rotation axis 18, attempting to rotate the knuckle-boom 14 in a clockwise direction. In the shown knuckle- boom crane 1 , the forces acting from the hoisting wire rope 30 on the knuckle-boom 14 at connection of the first guiding sheave 34 contributes to a negative moment S2 about the second rotation axis 18, and as such at least partially contributes to balancing out the moment acting of the knuckle-boom 14 around the second rotation axis from any load suspended from the hoisting wire rope 30. The necessary force needed to be supplied by the knuckle-boom rotation means 42 in order to lift the load by rotation of the knuckle-boom 14 may thus be significantly reduced. The placement of the first guiding sheave 34 on the first portion 20 of the knuckle-boom 14 defines the moment arm for the forces acting from the hoisting wire rope 30 on the second rotation axis 18. In an alternative embodiment, the first guiding sheave 34 may be movably/slidably connected to the knuckle-boom 14 so as to possibly tailor and optimize the moment arm and thus the negative moment for different loads, as discussed above.

Figure 2 shows the knuckle-boom crane 1 in a second position of use, wherein a knuckle-boom folding means 53, here shown in the form of a folding wire rope 54 and a folding winch 55 is shown. In the figure, a pair of protective contact elements 56, 58 are shown on the knuckle-boom 14 and main boom 8, respectively, for preventing a collision/hard landing between be booms 8, 14 upon folding the knuckle-boom.

Figure 3 shows a vessel 6 provided with a knuckle-boom crane 1 according to the present inven- tion.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conju- gations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

C l a i m s

1. Knuckle-boom crane (1 ) for offshore hoisting operations comprising:

- a knuckle-boom crane support means (2, 4);

- a main boom (8) with a proximal end (12) and a distal end (16), the proximal end (12) being rotatably connected to the knuckle-boom crane support means (2, 4) at a first rotation axis (10);

- a main boom luffing means (13) for rotating the main boom (8) around the first rotation axis (10);

- a knuckle-boom (14) rotatably connected to the distal end (16) of the main boom (8) at a second rotation axis (18), the knuckle-boom (14) comprising a first portion (20) including a first end (22) and a second portion (24) including a second end (26);

- a load hoisting means (28), the load hoisting means (28) including an elongated hoisting member (30) and an elongated hoisting member drive means (32), a first end (34) of the elongated hoisting member (30) being connected to the elongated hoisting member drive means (32) and a second end (36) of the elongated hoisting member (30) being connect- able to a load suspension member; and

- a knuckle-boom rotation means (42) for rotating the knuckle-boom (14) around said second rotation axis (18), wherein

- the second rotation axis (18) is provided between the first and second portions (20, 24) of the knuckle-boom (14);

- the elongated hoisting member (30) is adapted to run over both said first portion (20) and said second portion (24) of the knuckle-boom (14) so that a force acting on the knuckle-boom from the elongated hoisting member (30) creates a positive moment (S1 ) on the second portion (24) of the knuckle-boom and a negative moment (S2) on the first portion (20) of the knuckle-boom (14); and

- the knuckle-boom rotation means (42) comprises an elongated tensioning member (44) and an elongated tensioning member drive means (46), a first end of the elongated tensioning member (44) being connected to the elongated tensioning member drive means (46) and a second end of the elongated tensioning member being

directly or indirectly connected to the knuckle-boom (14) at said first portion (20) thereof, c h a r a c t e r i z e d i n that the elongated tensioning member drive means (46) is connected to the main boom (8).

2 Knuckle-boom crane (1 ) according to claim 1 , wherein the knuckle-boom rotation means (42) comprises two elongated tensioning member drive means (46).

3. Knuckle-boom crane (1 ), according to any of the claims 1 or 2, wherein the elongated tensioning member (44) is connected to the knuckle-boom towards the first end thereof.

4. Knuckle-boom crane (1 ) according to any of the claims 1 -3, wherein the knuckle-boom crane (1 ) further is provided with a knuckle-boom folding means (53).

5. Knuckle-boom crane (1 ) according to claim 4, wherein the knuckle-boom folding means (53) comprises an elongated folding member (54) and an elongated folding member drive means (55) connected to the main boom (8), the elongated folding member (54) having a first end connected to the elongated folding member drive means and a second end connected to the knuckle-boom (14) at the second portion (24) thereof.

6. Knuckle-boom (1 ) crane according to any of the preceding claims, wherein the knuckle- boom rotation means (42) comprises a motor, such as an electric motor, provided around the second rotation axis (18).

7. Knuckle-boom crane (1 ) according to any of the preceding claims, wherein the second rotation axis (18) is provided substantially mid-way between the first and second ends (20, 24) of the knuckle-boom (14).

8. Knuckle-boom crane (1 ) according to any of the preceding claims, wherein the knuckle- boom (14), at the first portion (20) thereof, is provided with a sheave (33) over which the elongated hoisting member (30) is adapted to run, the sheave (33) being movably connected to the knuckle-boom (14).

9. Vessel (6) comprising a knuckle-boom crane (1 ) according to any of the preceding

claims.

10 Method for operating a knuckle-boom crane (1 ) according to claim 1 , c h a r a c t e r i z e d i n that the method comprises the steps of:

- operating the elongated hoisting member drive means (28) so as to regulate the moment about the second rotation axis (18); and

- regulating the moment about the second rotation axis (18) so as to maintain a positive moment (S1 ) around the second rotation axis independent of the load suspended from the second end (36) of the elongated hoisting member (30).