NL2020664B1 - Offshore crane - Google Patents

Abstract

The invention relates to an offshore crane compensated for local water movement. The crane has a substructure, a boom, and a heave device comprising a winch, a hoisting cable, and a gripper for carrying a load. The boom has a fixed end supported by the substructure and a free end. The hoisting cable extends from the winch, along the boom to the free end of the boom, and suspends from the free end of the boom freely to the gripper. At the fixed end, the boom is configured for slewing with respect to the substructure around a slewing axis and for luffing with respect to the substructure around a luffing axis, the luffing axis being perpendicular to the slewing axis. In order to compensate for local water movement, an xaxis translational movement, y-axis translational movement, z-axis translational movement, xaxis rotational movement, y-axis rotational movement and z-axis rotational movement are measured; on the basis of these measurements an x-, y-, and z-translation, experienced by the free end of the boom due to movement of the substructure, are determined; and these determined x-, y-, and z-translations are neutralized by correspondingly actuating the actuator system such that the position of the gripper is kept motionless. The invention further relates to a method for compensating such crane for local water movement.

Description

© 2020664 © B1 PATENT (2?) Application number: 2020664 © Application submitted: March 26, 2018 © Int. Cl .:

B66C 23/52 (2018.01) B66C 13/02 (2018.01) B63B

27/10 (2019.01)

Application registered:

October 2019 © Patent holder (s):

Barge Master IP B.V. in Rotterdam.

(43) Application published:

© Inventor (s):

Jorrit Lennart van Dommelen in ROTTERDAM.

Patent granted:

October 2019 © Patent issued:

October 2019 © Authorized representative:

ir. J.C. Volmer et al. In Rijswijk.

54) Offshore crane

7τ) The invention relates to an offshore crane compensated for local water movement. The crane has a substructure, a boom, and a heave device including a winch, a hoisting cable, and a gripper for carrying a load. The tree has a fixed end supported by the substructure and a free end. The hoisting cable extends from the winch, along the tree to the free end of the tree, and suspends from the free end of the tree freely to the gripper. At the fixed end, the tree is configured for slewing with respect to the substructure around a slewing axis and for respecting with the substructure around a luffing axis, the luffing axis being perpendicular to the slewing axis. In order to compensate for local water movement, an x-axis translational movement, y-axis translational movement, z-axis translational movement, x-axis rotational movement, y-axis rotational movement and z-axis rotational movement are measured; on the basis of these measurements an x, y, and z translation, experienced by the free end of the tree due to movement of the substructure, are determined; and these determined x, y, and z translations are neutralized by correspondingly actuating the actuator system such that the position of the gripper is kept motionless. The invention further relates to a method for compensating such a crane for local water movement.

NL B1 2020664

This patent has been granted regardless of the attached result of the research into the state of the art and written opinion. The patent differs from the documents originally submitted. All submitted documents can be viewed at the Netherlands Patent Office.

P33528NL00 / YGR

Title: Offshore crane

The present invention relates to an offshore crane.

When transferring loads from a vessel to another vessel or to some other construction, which may be movable or unmovable relative to the ground, problems arise due to movement of the water on which the vessel floats. Motion of the water subjects the crane, and consequently the load to be transferred, to similar movements. In case the load is carried by a hoisting cable, the water motion will cause swinging or the cable and load. Similar problems arise when a crane is delivering a load onto the vessel or the crane (i.e. the vessel onto which the crane itself is mounted). In general, movement of the water causes the vessel to move, which in turn causes movement of the crane, movement of the location on the vessel which is to receive the load, and swinging of the cable and load.

Also when the weather conditions are very calm, the problems mentioned above due to local water movement are present. In this respect it is noted that although the water is obviously brought in by strong wind, the effects of wind can lag for weeks in water and have influence on water at a large distance away from the location of the wind. Even the water may look like very calm, but still being in motion due to wind weeks ago and / or far away. The effect of this for example marine building operations is that one has to wait for the water to be almost motionless, in case for example a crane with hoisting cable is used safely.

With respect to the motions to which a vessel is subject to water, it is noted that a vessel is in fact subject to 6 degrees of freedom of movement, three translational movements and three rotational movements. Using a mathematical approach based on a carthesian coordinate system having an imaginary set of three orthogonal axes - an x-axis, y-axis and zaxis - these 6 movements can be called x-axis translational movement, y-axis translational movement, z- axis-translational movement, x-axis rotational movement, y-axis rotational movement and z-axis rotational movement. It is to be noted that from a mathematical point of view there are also other equivalent manners to define the 6-degrees of movement in a space, for example the 3 axes used may not be orthogonal with respect to each other or a so called spherical coordinate system may be used. It is just a matter of mathematical calculation to transfer one definition of 6 degrees of freedom of movement into another definition of 6 degrees of freedom of movement. Using the so called carthesian coordinate

-2system and defining the z-axis extending vertically, the x-axis extending in the longitudinal direction of a vessel and the y-axis extending in the transverse direction of a vessel, the x-axis translational movement is in practice called surge the y-axis translational movement is in practice called sway the z-axis translational movement is in practice called heave the x-axis rotational movement is in practice called roll the y-axis rotational movement is in practice called pitch the z-axis rotational movement is in practice called yaw.

With offshore cranes it is desired to compensate for movement of the vessel, caused by local water movement, in order reduce the impact of movement of the vessel on the load.

Possibly the eldest manner of compensation is the so called "heave compensation". With heave compensation only the hoisting cable is used for compensation. The principle of heave compensation is that the cable of the hoisting system is extended or shortened in order to maintain the load on a vertical height. Swinging of the cable and load is not prevented.

Another way of compensation is using a so called Stewart platform carrying the crane. A Stewart platform is a triangular platform supported by three pairs of two hydraulic actuators which are capable of controlling all 6 degrees of freedom of movement of the platform. Theoretically this allows for a control which can keep the platform completely motionless with respect to the fixed world (while the vessel is moving underneath the motionless platform). When the platform is kept motionless, also the crane is kept motionless. Stewart platforms are large, heavy, complex and require very high power for operating the platform. Not only the Stewart platform is complex, also its control is complex. A further disadvantage of such a platform is that the platform is so to say an additional unit to be placed on the vessel between the vessel and the crane.

A simplified platform for compensating a crane for water motion is known from

WO2010 / 114359. Also this simplified platform is kept motionless with respect to the fixed world (while the vessel is moving underneath the motionless platform). Although this platform is capable of keeping a crane almost fully motionless with a simple control, the platform is still large and heavy.

According to WO2014 / 123414, the footprint is - with respect to WO2010 / 114359 considerably reduced by a pedestal-like construction or the carrying platform. This construction is still large in height and heavy. Further in WO2014 / 123414 the power used for

-3controlling the actuators is still considerable. Also a further disadvantage of this platform is that the platform is so to say an additional unit to be placed on the vessel between the vessel and the crane.

Further, WO 2017/103139 shows a knuckle boom crane which is provided with an additional accessory carried by the free end of the knuckle boom. The accessory - not the knuckle boom crane itself - is controlled by a motion compensation system in order to keep the load carried by a hook on a hoisting cable stationary. For this purpose, the accessory has a telescopic boom, which is rotatably mounted to the free end of the knuckle boom crane for rotation around two mutually perpendicular rotation axes and which is provided with a heave device including a winch, a hoisting cable and a crane hook. To keep the crane hook stationary, the telescopic boom is operated for i) rotation around said two mutually perpendicular rotation axes and ii) extended or shortened, and additionally the heave device is operated. This accessory is heavy and introduces additional large momentums and forces to be carried by the knuckle boom crane on top of the load.

The present invention has as its object to provide an alternative offshore crane with motion compensation, which crane preferably overcomes one or more of the drawback or the above prior art.

This object is according to the invention achieved by providing an offshore crane according to claim 1. This crane comprises a substructure and a superstructure. The superstructure consists of a base, a tree structure, and a heave device. The tree structure has a fixed end supported by the base and a free end. The heave device comprises a winch, a hoisting cable, and a gripper configured for carrying a load. The winch can for example be mounted on the base, but it is also very well conceivable that the winch is provided on the tree, like on the free end of the tree or at a distance from the free end of the tree. The hoisting cable extends from the winch to the gripper and suspends from the free end of the tree structure freely to the gripper. This part of the hoisting cable between the free end of the tree structure and the gripper, is the part, hanging freely downwards under the influence of gravity, which is in practice subject to swinging motions due to movement of the vessel. In other words, this part of the cable suspends freely from the free end of the gripper to the gripper. The gripper may be a simple crane hook, but can - depending on circumstances and requirements - also be or more complicated structure. The crane comprises a slewing actuator and a luffing actuator. The slewing actuator is configured for slewing the base with respect to the substructure around a slewing axis. At the fixed end of the tree structure the luffing actuator is provided. The luffing actuator is configured for luffing the boom structure

-4with respect to the base around a luffing axis, the luffing axis being perpendicular to the slewing axis. Assuming the substructure of the crane is oriented in its neutral position (i.e. not tilted due to movement of the vessel or water), the slewing axis will extend essentially vertical.

According to the invention the substructure of the crane will be immovably mounted on the vessel, i.e. the substructure is immovable with respect to the vessel.

In order to describe the crane according to the invention further, an imaginary set of orthogonal axes is defined, including an x-axis, an y-axis and a z-axis, extending the z-axis extends vertically. This imaginary set of orthogonal axes may be defined with respect to the vessel or the substructure of the crane or any part of the superstructure of the crane, like the boom structure of the crane.

In order to compensate for local water movement, the crane according to the invention furthermore includes a motion compensation system. This motion compensation system comprises a sensor system, an actuator system and a control system.

The sensor system is configured for sensing x-axis translational movement, y-axis translational movement, z-axis translational movement, x-axis rotational movement, y-axis rotational movement and z-axis rotational movement. These movements may be sensed with respect to a reference point. This reference point can for example be the fixed world (which does not move), but the reference point can also be some other point, for example another vessel. But a reference point is not required when the motion compensation system senses the changes in x-axis translational movement, y-axis translational movement, z-axis translational movement, x-axis rotational movement, y-axis rotational movement and z-axis rotational movement.

It is practical to arrange the sensor system on the substructure of the crane. But taking into account that the substructure of the crane will be immovable with respect to the vessel, the sensor system may equally well be arranged on the vessel. It is equally conceivable that a sensor system has already been used on the vessel. Further it is noted that the sensor system may also be provided on a moving part of the crane, such as on the boom structure of the crane.

The sensor system is further configured for generating sensor signals representing said sensed movements of the substructure.

-5The actuator system is configured for manipulating the position of the gripper.

The control system is adapted to generate control signals for driving the actuator system in response to the sensor signals such that the position of the gripper is compensated for said sensed movements.

In order to transfer signals between the sensor system, the control system and actuator system, the control system is connected to the sensor system for receiving the sensor signals from the sensor system and the control system is connected to the actuator system for sending the control signals to the actuator system. These connections may be wired connections but may also be wireless.

The control system is according to the invention adapted to determine from the sensor signals an x, y, and z translation experienced by the free end of the tree structure or gripper due to said sensed movements. In general these sensed movements will be the movements sensed by the substructure. The control system is further adapted to generate, on the basis of these determined x, y, and z translations, control signals for driving the actuator system such that the position of the gripper is kept stationary, ie the position of the gripper does not change essentially, with respect to a predetermined target location. This predetermined target location can be a location on the fixed world, a location on the vessel carrying the crane or a location on another vessel.

According to the invention the substructure of the crane is mounted directly on the vessel and immovable with respect to vessel. Here, "mounted directly on the vessel" means that there is no intermediate construction allowing substantial movement or the substructure with respect to the vessel. Instead of using an intermediate construction, like a Stewart platform or a platform according to WO-2010/114359 or WO-2014/123414, for keeping the crane motionless, the control system according to the invention does not neutralize movement of the crane due to movement of the vessel but it neutralizes only x, y and z translations which the free end of the tree structure or the gripper would experience due to movement of the vessel. These x, y, and z translations which are the free end of the tree structure would experience (in absence of compensation) are caused by a) x-axis translation, y-axis translation and z-axis translation of the vessel and by b) x-axis rotation, y-axis rotation and zaxis rotation of the vessel. So the control system according to the invention calculates, from these X, y, and z translations of the vessel and the x, y, z rotations of the vessel, the x, yand z-translation which the gripper / free end of the boom structure would, in case of absence of motion compensation, experience due to the x, y, and z translations of the vessel and the

-6x, y, z rotations of the vessel. Subsequently, the control system according to the invention generates control signals for driving the actuators such that only x, y and z translation felt by the gripper or free end of the tree structure are neutralized.

Doing so considerable less power for the actuators is required because only the mass of the boom structure and load is moved instead of - as is the case in the prior art - the mass of the entire crane (including its substructure) and a platform carrying the crane. Further, the control is very much simplified because rotations of the vessel as such are not compensated but only the translations caused by these rotations are compensated. This simplification of the control is based on the insight that the cable carries the load does not transfer x-axis, yaxis and z-axis rotation of the vessel to the load in the case the free end of the boom structure is kept vertically above the load and that the vertical height of the load can be kept stationary by manipulating the vertical height of the free end of the tree structure and / or manipulating the heave device. Further, x-axis, y-axis and z-axis rotation experienced by the free end of the tree structure in practice turn out to be negligibly small.

Taking into account that according to the invention the movement of the crane itself - ie the movement of the boom structure and optionally the movement of the load under the influence of the heave device, can be used for neutralizing the effects of movement of the vessel - the actuator system of the motion compensation system thus comprises the slewing actuator configured for slewing the base with respect to the substructure around the slewing axis and the luffing actuator configured for luffing the tree structure with respect to the base around the luffing axis. Supplementary, the actuator system or the motion compensation system may further include a heave actuator configured for paying out or hauling in the hoisting cable.

According to a further embodiment of the crane according to the invention, the actuator system comprises a slewing actuator configured for actuating the base to rotate around the slewing axis with respect to the substructure, and a luffing actuator configured for actuation the boom structure to rotate around the luffing axis with respect to the base; in which the control system is adapted to generate a slewing control signal for driving the slewing actuator; the control system is adapted to generate a control signal for driving the control actuator.

According to another further embodiment of the crane according to the invention, the boom structure is a knuckle boom having a first boom member and a second boom member; fall the first and second boom member are connected to each other by a knuckle hinge

-7configured for knuckling the first and second tree member with respect to each other around a knuckle axis, the knuckle axis being parallel to the luffing axis. A knuckle tree allows for a large range in which z-translations and y-or x-translations can be neutralized.

According to another further embodiment of the crane according to the invention with knuckle boom, the actuator system further comprises a knuckle actuator configured for actuating the first and second boom member to rotate with respect to each other around the knuckle axis; The control system is adapted to generate a knuckle control signal for driving the knuckle actuator.

In case of a knuckle tree, the position of the gripper can be kept stationary by keeping the position of the free end of the tree structure stationary. So in case the tree structure is a knuckle tree, the control system may be adapted to determine from the sensor signals an x, y, and z translation experienced by the free end of the tree structure due to said sensed movements, and to generate, on the basis of these determined x, y, and z translations, control signals for driving the actuator system such that the position of the free end of the tree structure is kept stationary with respect to a predetermined target location.

According to another further embodiment of the crane according to the invention, the actuator system further comprises a heave actuator configured for paying out or hauling in the cable; in which the control system is adapted to generate a heave control signal for driving the heave actuator. Using the heave actuator allows for a large range in which z-translations can be neutralized. The heave actuator may actuate the winch to pay out or haul in the cable, but it is also conceivable that the heave actuator acts in different ways on the cable in order to pay out or haul in the cable. For example, the hoisting cable may have a first cable part run from the winch to the gripper and a second cable part returning from the gripper to an attachment point where the end of the second cable part is attached to the crane, with the heave actuator acts on the attachment point for moving this attachment point and consequently hauling in or paying out the hoisting cable.

According to a second aspect, the invention relates to an assembly including an offshore crane according to the invention and a vessel, the substructure of the crane is mounted on the vessel such that it is immobile with respect to the vessel.

According to a third aspect, the invention relates to a method for compensating an offshore crane for local movement, the offshore crane has a substructure and a superstructure, the superstructure including a base, a boom structure, and a heave device

-8 maintain a winch, a hoisting cable, and a gripper configured for carrying a load. The tree structure has a fixed end supported by the base and a free end. The hoisting cable extends from the winch, along the tree structure to the free end of the tree structure, and from the free end of the tree structure freely to the gripper. The base is configured for slewing with respect to the substructure around a slewing axis. At the fixed end, the tree structure is configured for luffing with respect to the base around a luffing axis, the luffing axis being perpendicular to the slewing axis. An x-axis, an y-axis and z-axis define an imaginary set of orthogonal axes, the z-axis extending vertically. The method according to the third aspect comprises the following steps:

- measuring an x-axis translational movement, y-axis translational movement, z-axis translational movement, x-axis rotational movement, y-axis rotational movement and z-axis rotational movement;

- determining - on the basis of the measured x-axis translational movement, y-axis translational movement, z-axis translational movement, x-axis rotational movement, yaxis rotational movement and z-axis rotational movement - an x-, y -, and z-translation experienced by the gripper or by the free end of the tree structure due to movement of the substructure, and

- neutralizing these determined x, y, and z translations by correspondingly actuating the actuator system such that the position of the gripper is kept motionless with respect to a predetermined target location, like a location on the fixed world, a location on the vessel carrying the crane or a location on another vessel.

In this application the term "adapted to" is used, especially in relation to the control system, with the meaning "configured for".

The present invention will be explained further with reference to the drawings. In these drawings:

- Figure 1 shows highly schematically a first embodiment of the invention;

- Figure 2 shows schematically compensation of a z-translation of the gripper or the crane according to figure 1;

- Figure 3 shows schematically compensation of an y-translation of the gripper of the crane according to figure 1;

- Figure 4 shows schematically compensation of an x-translation of the gripper of the crane according to figure 1;

- Figure 5 shows highly schematically a second embodiment of the invention;

- Figure 6 shows schematically compensation of a z-translation of the gripper or the crane according to figure 2; and

-9- Figure 3 shows schematically compensation of an y-translation of the gripper of the crane according to figure 2.

As indicated with the orthogonal set of axes in the drawings, the z-direction is - in these drawings - defined as the vertical direction, the y-direction is - in these drawings - defined as the horizontal direction transverse to the length direction of the vessel and the x-direction is in these drawings - defined as the horizontal direction parallel to the vessel.

The Figures 1-7 show a vessel 1 provided with a crane 2 according to the invention. In the two examples or figures 1-7, the crane is a pedestal crane 2, but the crane can also be of another type. The crane 2 has a substructure 3 carrying a base 27. The base 27 in turn carries an operator cabin 4, a boom structure 5, 6, and a heave device 7, 8, 9. In case of a pedestal crane, the substructure is also called 'pedestal'.

The tree structure 5, 6 has a fixed end 10 supported by the base 27 and a free end 11.

The heave device comprises a winch 7, a hoisting cable 8 and a gripper 9. The gripper may be a crane hook 9 as shown in the figures 1-7, but depending on the load 28 to be carried by the gripper any other type of gripper is very well conceivable. In case the load 28, an example is a sea container, the gripper may include a frame provided with four corner casting locks to be locked to the four upper corners of the sea container. In the examples shown, the winch 7 is mounted on the base 27 and the hoisting cable 8 extends from the winch 7 along the boom structure 5, 6 to the free end 11 of the boom. At the free end 11, the part 12 of the cable suspends downwards from the free end 11 and carries the gripper 9. However the winch 7 may also be mounted on the free end 11 of the tree structure or at any other place of the tree structure.

The base 27 is configured for slewing - see arrow S - with respect to the substructure 3 around slewing axis 13. In neutral position, as shown in the figures 1-7, the slewing axis 13 extends vertically, parallel to the z-direction. But when the vessel is subject to roll - i.e., rotational movement around the x-axis - and / or subjected to pitch - i.e., rotational movement around the y-axis - the slewing axis will assume a slanting position with respect to the vertical. In order to slew the base 27 with respect to the substructure 3 a slewing actuator 14 is provided. This slewing actuator 14 may be an example of an electric or hydraulic drive provided in the substructure 3 and configured to rotate the base 27 with respect to the substructure 3 around the slewing axis 13.

- 10At the fixed end 11, the tree structure 5, 6 is configured for luffing with respect to the base 27 around luffing axis 15. In neutral position, as shown in the figures 1-7, the luffing axis 15 extends horizontally. In figures 1, 2, 3, 5, 6 and 7 the luffing axis extends perpendicular to the sheet of paper. But when the vessel is subject to pitch - i.e. rotational movement around the y-axis - the luffing axis 15 will assume a slanting position with respect to the horizontal. In order to luff the tree structure 5, 6 with respect to the base 27 a luffing actuator 16 is provided. This luffing actuator 16 may for example be an hydraulic cylinder-piston assembly extending between the base 27 and the first boom member 5 of the tree structure and configured to rotate the first boom member 5 with respect to the base 27 around the luffing axis 15 by extending or retracting the piston-cylinder assembly in the direction of double arrow L.

The winch 7 is provided with a winch actuator 17 configured for actuating the winch for hauling in or paying out the cable 8 in order to raise or lower the gripper 9.

The figures 1-7 show a first edition of the crane according to the invention - see figures 1-4 - and a second edition of the crane according to the invention - see figures 5-7. The difference between these two is essentially in the tree structure.

In the first embodiment of the crane according to the invention as shown in figures 1-4, the boom structure is a knuckle boom including a first boom member 5, also known as the main boom, and a second boom member 6, also known as jib. The first boom member 5 and second boom member 6 are connected to each other by a knuckle hinge 18 for knuckling the first boom member 5 and second boom member 6 around a knuckle axis 19 with respect to each other. In neutral position, as shown in figures 1-7, the knuckle axis 19 extends horizontally. In figures 1, 2, 3, 5, 6 and 7 the knuckle axis 19 extends perpendicular to the sheet of paper. But when the vessel is subject to pitch - i.e. rotational movement around the y-axis - the knuckle axis 19 will assume a slanting position with respect to the horizontal. In order to knuckle the second boom member 6 with respect to the first boom member 5 a knuckle actuator 20 is provided. This knuckle actuator 20 may for example be an hydraulic cylinder-piston assembly extending between the first boom member 5 and the second boom member 6 and configured to rotate the second boom member 6 with respect to the first boom member 5 around the knuckle axis 19 by extending or retracting the piston-cylinder assembly in the direction of double arrow K.

In the second embodiment of the crane according to the invention as shown in figures 5-7, the boom structure is a single boom including a first boom member 5.

- 11 Now returning to the according to figures 1-7 in general, the crane according to the invention comprises a sensor system 21 and a control system 22. The slewing actuator 14, the luffing actuator 16, the optional knuckle actuator 20, the optional winch actuator 17 and optionally further actuator form together an actuator system configured for manipulating the position of the gripper 9. The sensor system 21, the actuator system 14, 16, 17, 20, and the control system 22 together form a motion compensation system .

The sensor system 21 may be provided on the vessel, on the substructure of the crane, or on any part of the superstructure of the crane. In the first embodiment shown in figures 1-4, the sensor system is provided by way of example on the substructure 3 of the crane. In the second embodiment shown in figures 5-7, the sensor system 21 is provided by way of example on the vessel 1. Similar applies to the control system 22. Also the control system 22 may be provided on the vessel, on the substructure of the crane, or any part of the superstructure of the crane. For ease of drawing, the control system 22 is shown in the first and second embodiment in figures 1-7 provided on the vessel.

The sensor system is configured for sensing movement in the x-direction, y-direction and zdirection (x-axis translational movement, y-axis translational movement and z-axis translational movement, respectively), and rotation around the x-axis, y -axis and z-axis (xaxis rotational movement, y-axis rotational movement and z-axis rotational movement, respectively) and for generating sensor signals representing said sensed movements / rotations. By means of connection 23 the sensor system 21 is connected to the control system 22 in order to transfer the sensor signals from the sensor system to the control system. This connection may be wired, wireless or by any other means allowing transfer of sensor signals from the sensor system 21 to the control system.

The control system 23 generates control signals for driving the actuator system in response to the sensor signals such that the position of the gripper is compensated for the movements sensed by the sensor system. By means of connections 24-27 the control system 22 is connected to the actuator system 14, 16, 17, 20 in order to transfer the control signals from the sensor system to the actuator system. These connections may be wired, wireless or by any other means allowing transfer of control signals from the control system 22 to (the actuators or) the actuator system. Connection 24 connects the control system 22 with the slewing actuator, connection 25 connects the control system 22 with the winch actuator, connection 26 connects the control system with the luffing actuator and connection 27 connects the control system with the knuckle actuator.

- 12With respect to figures 2-4 it is noted that the sensor system, the control system and the connections 23-27 are not shown in order to avoid these drawings from getting clouded with too many lines, but in figure 1 these are all shown . With respect to figures 6-7 it is noted that the sensor system, the control system and the connections 23-26 are not shown in order to avoid these drawings from getting clouded with too many lines, but in figure 5 these are all shown.

X, y and z translation experienced by the free end of the tree structure due to the sensed movements of the vessel or substructure or any other part of the crane, depending on where the sensor system is arranged. The sensed movements consist of an x-axis translational movement, y-axis translational movement, z-axis translational movement, x-axis rotational movement, y-axis rotational movement and z-axis rotational movement. These movements represent the movement to which the vessel is subject to a moment in time by the water motion. Depending on the movement to which the vessel is being subjected, one or more of the x-axis translational movement, y-axis translational movement, z-axis translational movement, x-axis rotational movement, y-axis rotational movement and z-axis rotational movement may be zero or have any other value. In absence of any movement compensation and other movement action of the crane, the movements sensed by the sensor system would result in an x, y and z translation experienced by the free end of the gripper. These x, yand z translations that the free end of the gripper would experience in absence of any movement compensation or other movement action of the crane, can be calculated from the movements sensed by the sensor system. The control system is programmed to make this calculation, more in general the control system is adapted to determine the x, y, and ztranslation experienced by the free end of the gripper. The control system subsequently uses these determined x, y and z translations as input for a control algorithm that generates control signals for driving the actuators or the actuator system to keep the position of the gripper stationary. Stationary here means that the position of the gripper with respect to a reference point, like the fixed world or a position on deck of the vessel, does not essentially change. For this purpose the free end of the tree is kept vertically above the load 28, ie the x and y position of the free end of the gripper with respect to the reference point is kept stationary, on the one hand, and the vertical height of the gripper with respect to the reference point is kept stationary, on the other hand. So instead of saying, that “the control system is adapted to generate, on the basis of these determined x, y, and z translations, control signals for driving the actuator system to keep the position of the gripper stationary with respect to a predetermined target location ”, one can say - as is the case in claim 1 that“ the control system is adapted to generate, on the basis of these determined x, y, and

- 13z-translations, control signals fordriving the actuator system to keep the x and y position or free end of the tree structure stationary with respect to a predetermined target location, on the one hand, and to keep b) the z-position or the gripper stationary with respect to said predetermined target location, on the other hand ”.

The control system will now be further illustrated with reference to the figures 2-4 and 5-6. In these figures the vessel and crane are shown in two different positions (in case of figure 4, three different positions), a first position shown in solid lines and a second - and in case of figure 4 a third - position shown in dashed lines .

Figure 2 shows the case that the vessel and knuckle boom crane or figure 1 are subject to only a z-translational movement caused by the water. In the first position - solid lines - the level of the water is higher than in the second position - dashed lines -. In both positions, the gripper 9 is at the same position with respect to the fixed world. This is achieved by, starting from the first position shown in solid lines, operating the luffing actuator 16 and knuckle actuator 20 to move the boom structure 5, 6 to the second position shown in dashed lines and / or operating the winch actuator to adjust the winch so that the z-position of the gripper is kept stationary. In the example of figure 2, the z-translational movement caused by the water is relatively small so that the winch actuator does not need to be actuated. In case of a larger z-translational movement caused by the water, the winch actuator may be required to be actuated as well because the tree structure alone is unable to eliminate the x, y and ztranslation experienced by the gripper 9.

Figure 3 shows the case that the vessel and knuckle boom crane or figure 1 are subject to only a y-translational movement caused by the water. The first position - solid lines - or the vessel is to the right of the second position - dashed lines - of the vessel. In both positions, the gripper 9 is at the same position with respect to the fixed world. This is achieved by, starting from the first position shown in solid lines, operating the luffing actuator 16 and knuckle actuator 20 to move the boom structure 5, 6 to the second position shown in dashed lines and / or operating the winch actuator to adjust the winch so that the z-position of the gripper is kept stationary. In the example of figure 3, the y-translational movement caused by the water can be fully neutralized by only actuating the luffing actuator 16 and knuckle actuator 20. In other circumstances, the winch actuator may be required to be actuated as well because the boom structure alone is unable to eliminate the x, y and z translation experienced by the gripper 9.

- 14 Figure 4 shows the case that the vessel and knuckle boom crane or figure 1 are subject to only x-translational movement caused by the water. The first position - in solid lines - or the vessel is midway between the second and third position - both in dashed lines - or the vessel. In the second position on the right, the vessel has moved in forward direction, and, in the third position, the vessel has moved in backward direction. In all positions, the gripper 9 is at the same position with respect to the fixed world. This is achieved by, starting from the first position shown in solid lines, operating the slewing actuator 14 to rotate the base 27 with respect to the substructure 3 and simultaneously operating the luffing actuator 16 and knuckle actuator 20 to move the boom structure 5, 6 with respect to the base 27 in the second or third position shown in dashed lines and / or by operating the winch actuator to adjust the winch so that the z-position of the gripper is kept stationary. In the example of figure 4, the x-translational movement caused by the water can be fully neutralized by only actuating slewing actuator 14, the luffing actuator 16 and knuckle actuator 20. In other circumstances, the winch actuator may be required to be actuated as well because the tree structure alone is unable to eliminate the x, y and z translation experienced by the gripper 9.

In the case of a knuckle boom crane as shown in figures 1-4, the gripper can be kept stationary by keeping the free end of the boom structure stationary. So in case of a knuckle boom crane, the end of claim 1 can be worded as follows the control system is adapted to generate, on the basis of these determined x, y, and z translations, control signals for driving the actuator system such that the position of the free end of the tree is kept stationary with respect to a predetermined target location.

Figure 6 shows the case that the vessel and crane or figure 5 are subject to only a transnational movement caused by the water. In the first position - solid lines - the level of the water is lower than in the second position - dashed lines -. In both positions, the gripper 9 is at the same position with respect to the fixed world. This is achieved by, starting from the first position shown in solid lines, operating the winch actuator 20 to pay out the cable 8 in order to increase the length of the cable part 12 suspending freely from the free end 11 of the boom member 5.

Figure 7 shows the case that the vessel and crane or figure 5 are subject to only a ytranslational movement caused by the water. The first position - solid lines - of the vessel is to the left of the second position - dashed lines - of the vessel. In both positions, the gripper 9 is at the same position with respect to the fixed world. This is achieved by, starting from the first position shown in solid lines, simultaneously operating the luffing actuator 16 and winch

- 15 actuator 17 to move the tree structure 5 to the second position shown in dashed lines. In the example of figure 7, the y-translational movement caused by the water can be fully neutralized by only actuating the luffing actuator 16 and winch actuator 17.

Referring to the figures 1-7, it is noted that, although these figures 1-7 all show a crane according to the invention arranged at a longitudinal of the vessel, it will be clear that the crane according to the invention can be equally well arranged at the star or bow of the vessel.

Claims (8)

CONCLUSIONS An offshore crane comprising a substructure and a superstructure, the superstructure comprising: a base, a boom construction, and a hoisting device comprising a winch, a hoisting cable, and a gripper carried by the hoisting cable adapted to carry a load; wherein the boom structure has a fixed end and a free end supported by the base; wherein the hoisting cable extends from the winch to the grab and hangs freely from the free end of the boom structure to the grab; wherein the crane comprises a rotary actuator and a swing actuator; wherein the rotary actuator is adapted to cause the base to rotate relative to the substructure about a rotary axis; wherein the pivot actuator is provided at the fixed end of the boom structure and is adapted to pivot the boom structure about a pivot axis relative to the base, the pivot axis being perpendicular to the axis of rotation; wherein an x-axis, a y-axis and a z-axis define an imaginary system of orthogonal axes, the z-axis of which extends vertically; wherein the crane further has a motion compensation system, comprising: a sensor system arranged o for detecting an x-axis translation movement, y-axis translation movement, z-axis translation movement, x-axis rotation movement, y-axis rotation movement and z-axis rotation movement, and o for producing sensor signals representative of those sensed movements, an actuator system adapted to manipulate the position of the gripper, and a control system that generates control signals for controlling the actuator system in response to the sensor signals such that the gripper is compensated for the sensed movements; wherein the control system is connected to the sensor system for receiving the sensor signals from the sensor system; wherein the control system is adapted to generate a rotary control signal for controlling the rotary actuator; wherein the control system is adapted to generate a swing control signal for driving the swing actuator; 17 wherein the control system is connected to the actuator system for sending control signals to the actuator system; wherein the motion compensation system actuator system comprises the rotary actuator and the swing actuator; and wherein the control system is equipped to determine an x, y, and z translation from the sensor signals that feels the free end of the boom structure as a result of those sensed movements, and to, based on this determined x, y and z-translations, to generate control signals for controlling the actuator system such that i) the x and y positions of the free end of the boom construction are held stationary relative to a predetermined target location, and that ii) the z position of the gripper is held stationary relative to a predetermined target location. 2. Offshore crane as claimed in claim 1, wherein the control system is equipped to generate - on the basis of those determined x, y and z translations - control signals which control the actuator system such that the free end of the boom construction is held stationary compared to a predetermined target location. An offshore crane according to any of claims 1-2, wherein the boom structure is a articulated boom with a first boom part and a second boom part; wherein the first and second boom members are connected to each other by a pivot hinge that is arranged to cause the first and second boom members to buckle relative to each other about a pivot axis, which pivot axis is parallel to the pivot axis. The offshore crane according to claim 3, wherein the actuator system of the motion compensation system further comprises a buckling actuator adapted to cause the first and second boom parts to rotate relative to each other about the buckling axis; and wherein the control system is adapted to generate a kink control signal for controlling the kink actuator. The offshore crane according to any of claims 1-4, wherein the actuator system of the motion compensation system further comprises a hoisting actuator adapted to celebrate or overtake the cable; and wherein the control system is equipped to generate a hoist control signal for controlling the hoist actuator. An assembly comprising an offshore crane according to any one of claims 1-6 and a vessel, wherein the substructure of the crane is mounted directly on the vessel such that the substructure is immobile with respect to the vessel. 7. Method for compensating offshore crane for local water movement, wherein the offshore crane has a substructure and a superstructure, the superstructure comprising: a base, a boom construction, and a hoisting device comprising a winch, a hoisting cable and a gripper adapted to carry a load; wherein the boom structure has a fixed end and a free end supported by the base; wherein the hoisting cable extends from the winch to the gripper and freely hangs down towards the gripper from the free end of the boom structure; wherein the base is adapted to pivot relative to the substructure about a pivot axis; wherein the boom structure, at the fixed end, is adapted to pivot relative to the base about a pivot axis, the pivot axis being perpendicular to the axis of rotation; wherein an x-axis, a y-axis and a z-axis define an imaginary system of orthogonal axes, the z-axis of which extends vertically; the method comprising the following steps: measuring an x-axis translation movement, y-axis translation movement, z-axis translation movement, x-axis rotation movement, y-axis rotation movement and z-axis rotation movement; determining - based on the measured x-axis translation movement, y-axis translation movement, zas-translation movement, x-axis rotation movement, y-axis rotation movement and z-axis rotation movement - an x, y and z translation felt by the free end of the boom structure due to movement of the substructure, and neutralizing these particular x, y and z translations by energizing the actuator system accordingly such that i) the x and y position of the free end of the boom structure are held stationary relative to a predetermined target location, and that ii) the z position of the gripper is held stationary relative to a predetermined target location. The method of claim 7, wherein said determined x, y, and z translations are neutralized by holding the free end of the boom assembly stationary relative to a predetermined target location.