The present invention relates to a crane for a vessel, comprising a main boom and an auxiliary boom having a smaller length than the main boom and being tiltable with respect to the main boom about a tilting axis, a hoisting cable which suspends from a cable guide at the auxiliary boom and a drive system for tilting the auxiliary boom with respect to the main boom in order to compensate for motion of a vessel.

Such a crane is known from NL 2010094. The known crane comprises an articulated jib and a hoisting cable which is guided via the jib. The jib parts are tiltable with respect to each other. One of the jib parts can be operated as a heave compensator by means of a control system whereas the other jib part is blocked.

An object of the invention is to provide a crane for accurate compensation of motion of a floating vessel.

This object is accomplished with the crane according to the invention, which is characterized in that the tilting axis forms a first tilting axis and the auxiliary boom is also tiltable with respect to the main boom about a second tilting axis extending transversely with respect to the first tilting axis, wherein the cable guide is located at a distance from the first and second tilting axes.

Due to this feature the auxiliary boom can be moved such that a hoist cable suspending from the auxiliary boom at the cable guide can be hold in place in horizontal direction when the crane is mounted on a floating vessel. Since the cable guide is located at a distance from both tilting axes, it follows circular paths during tilting actions, but this means that it can be displaced in different horizontal directions. As a consequence the suspending hoisting cable can be kept at a fixed horizontal position. This

relatively simple structure for compensating motions of the vessel .

specific embodiment the first and second tilting axes extend transversely with respect to the

longitudinal direction of the main boom. More particularly, the first tilting axis and the second tilting axis may extend perpendicularly to each other.

In a practical embodiment the first tilting axis and the second tilting axis cross each other. This means that one of the tilting axes can rotate about the other one. It is also conceivable that the tilting axes lie in a common plane.

The crane may comprise a base to which the main boom is movably mounted and wherein the first and second tilting axes are located remote from the base. In this case the main boom may be locked at a reference position and under operating conditions the auxiliary boom can be tilted with respect to the locked auxiliary boom. Since the auxiliary boom is shorter than the main boom it is relatively easy to compensate motions of the base by means of tilting the auxiliary boom.

The auxiliary boom can be tiltable with respect to the main boom about the first and second tilting axes when the main boom has a fixed position, for example, in case the crane comprises a base to which the main boom is movably mounted and wherein the first and second tilting axes are located remote from the base as described above, when the main boom is locked at a reference position. This means that the auxiliary boom is tiltable with respect to the main boom about the first and second tilting axes independent from any motion of the main boom.

The cable guide may comprise a boom tip sheave, but alternative guiding means at alternative locations of the auxiliary boom are conceivable. The crane may be provided with two wires for tilting the auxiliary boom with respect to the main boom, wherein the wires are mounted to the auxiliary boom at respective wire mounting locations which are located at a distance from the tilting axes and guided via the main boom to respective drivers for pulling the wires such that each of the wires is angled with respect to a plane in which the corresponding wire mounting location and the first tilting axis lies and with respect to a plane in which the

corresponding wire mounting location and the second tilting axis lies. This is a relatively simple drive system, but alternative drive systems are conceivable.

In order to keep a load suspending from the hoisting cable also in a fixed vertical position the crane may be provided with a hoisting cable control system for compensating vertical motion of the cable guide. This can be a standard heave compensation system.

The invention is also related to a vessel which is provided with a crane as described hereinbefore.

The invention is also related to a method of operating the crane on a vessel as described hereinbefore, wherein the main boom is set at a fixed position with respect to the vessel and the auxiliary boom is set in a neutral starting position in which one of the tilting axes extends substantially horizontally and perpendicularly to the

longitudinal direction of the auxiliary boom and in which a plane through the cable guide and said tilting axis is angled with respect to a horizontal plane, after which the auxiliary boom is tilted with respect to the main boom about the first and second tilting axes for maintaining the cable guide in a fixed horizontal position in response to motion of the vessel. This method serves to move the cable guide in

opposite horizontal directions with respect to the main boom. The invention will hereafter be elucidated with reference to very schematic drawings showing an embodiment of the invention by way of example.

Fig. 1 is a perspective view of a part of an embodiment of a crane according to the invention.

Fig. 2 is a similar view as Fig. 1 of a portion of the crane on a larger scale.

Fig. 3 a similar view as Fig. 1 as seen from above.

Fig. 1 shows a part of an embodiment of a crane 1 according to the invention. The crane 1 comprises a main boom 2 and an auxiliary boom 3. In the embodiment as shown in Fig.

1 the main boom 2 has a vertically oriented longitudinal direction. The angle between the centre line of the main boom

2 and the vertical is relatively small in this case, but may be larger in an alternative embodiment. The auxiliary boom 3 forms a boom tip and the length of the auxiliary boom 3 is shorter than the length of the main boom 2. In the situation as shown in Fig. 1 the centre lines of the auxiliary boom 3 and the main boom 2 are angled with respect to each other. The weight of the auxiliary boom 3 is much smaller than the weight of the main boom 2. The main boom 2 is movably mounted to a base (not shown) . For example, the main boom 2 may be rotatable with respect to the base about a vertically

oriented axis of rotation. The base may be fixed to a vessel where the crane 1 can be used for holding a load at a site where activities take place on the water, for example off ^¬ shore operations. The crane 1 is configured to keep the load at a fixed place in spite of motions of a floating vessel to which it is mounted.

The crane 1 is provided with a lifting/lowering line system comprising a hoisting cable 4 including a hook 5. The hoisting cable 4 is guided along a cable guide in the form of a boom tip sheave 6, the auxiliary boom 3 and the main boom 2 to a winch (not shown) . The auxiliary boom 3 is tiltable with respect to the main boom 2 about a first tilting axis 7 and a second tilting axis 8, see Fig. 2. The first and second tilting axes 7, 8 extend transversely with respect to the longitudinal direction of the main boom 2. Furthermore, the first and second tilting axes 7, 8 extend perpendicularly with respect to each other. In the embodiment as shown in Figs. 1 and 2 the first tilting axis 7 and the second tilting axis 8 cross each other, but in an alternative embodiment the axes 7, 8 may lie in a common plane. Comparing Figs. 1 and 2 it can be seen that the auxiliary boom 3 has different rotational positions about the first tilting axis 7. Due to this

configuration the boom tip sheave 6 can follow circular paths about the first tilting axis 7 in planes that extend

perpendicularly to the first tilting axis 7. Similarly, the boom tip sheave 6 can follow circular paths about the second tilting axis 8 in planes that extend perpendicularly to the second first tilting axis 8. The latter planes are rotatable about the first tilting axis 7. This configuration provides the opportunity to move the boom tip sheave 6 in different horizontal directions. In other words, the boom tip sheave 6 is moveable within a plane as seen from above. Fig. 3

illustrates the position of the auxiliary boom 3 which is rotated about the first tilting axis 7 as seen from above.

During the movements as described the vertical position of the boom tip sheave 6 will vary, as well.

Consequently, the hook 5 tends to move in vertical direction. This is compensated by means of a hoisting cable control system (not shown) , which is known in itself, for example an active heave compensation system or a constant cable tension system.

The crane 1 is provided with a drive system for tilting the auxiliary boom 3 with respect to the main boom 2 in order to compensate for motion of a floating vessel. In the embodiment as shown the drive system comprises two wires 9 which are fixed to the auxiliary boom 3. The wires 9 are guided by sheaves 10 on the main boom 2 where they are connected to respective drivers, for example hydraulic cylinders (not shown) . A control system for controlling the hydraulic cylinders may be configured such that upon

simultaneously retracting or extending the cylinders the auxiliary boom 3 will be tilted about the second tilting axis 8, hence moving the boom top sheave 6 in a horizontal

direction within a plane that extends perpendicularly to the second tilting axis 8. Upon simultaneously retracting one of the cylinders and extending the other one of the cylinders the auxiliary boom 3 can be tilted about the first tilting axis 7, hence moving the hook 5 also in a horizontal

direction but within a plane which extends perpendicularly to the first tilting axis 7. It may be clear that a lot of intermediate horizontal positions of the hoisting hook 5 are possible .

The wires 9 are fixed to the auxiliary boom 3 at respective wire mounting locations 11 which are located at a distance from the first and second tilting axes 7, 8. Each of the wires 9 is guided from its wire mounting location 11 to the corresponding hydraulic cylinder such that it is angled with respect to a plane in which its wire mounting location 11 and the first tilting axis 7 lies and also with respect to a plane in which its wire mounting location 11 and the second tilting axis 8 lies. This provides the opportunity to create a torque about the first and second tilting axes 7, 8. In the embodiment as shown in Figs. 1-3 the wires 9 are guided via sheaves 10 which are mounted on extensions at the upper side of the main boom 2. The sheaves 10 are located at a distance from the respective planes in which the wire mounting

locations 11 and the respective first and second tilting axes 7, 8 lie.

In a practical situation on sea the crane 1 is first set in a reference position, in which the hook 5 has a desired location in a horizontal plane, for example for holding a load at an off-shore installation. In the reference position the main boom 2 can be locked in a fixed position with respect to the vessel, whereas the auxiliary boom 3 can be set in a neutral starting position with respect to the main boom 2. Then, starting from the neutral starting

position the auxiliary boom 3 can be tilted about the tilting axes 7, 8 with respect to its neutral starting position in order to compensate motions of the floating vessel. The drive system can be controlled in response to sensor signals which detect motions of the vessel. Tilting of the auxiliary boom 3 may also cause a vertical displacement of the hook 5 with respect to a fixed point in vertical direction. In other words, the hook 5 may stay in a fixed point within the horizontal plane, but its position in vertical direction may vary. As described above this can be compensated by means of a known heave compensation system. In the neutral starting position, when the second tilting axis 8 extends

substantially horizontally, the plane through the boom top sheave 6 and the second tilting axis 8 should be angled with respect to a horizontal plane in order to allow the hook 5 to move in opposite horizontal directions. If the plane through the boom top sheave 6 and the second tilting axis 8 in the neutral starting position was also horizontal, the hook 5 would be movable in only one horizontal direction away from its starting position upon tilting the auxiliary boom 3 about the second tilting axis 8.

From the foregoing, it will be clear that the invention provides a crane for a vessel which provides the opportunity to keep a load at a fixed location in horizontal as well as in vertical direction.

The invention is not limited to the embodiment shown in the drawings and described hereinbefore, which may be varied in different manners within the scope of the claims and their technical equivalents.