WO2023020936A1

WO2023020936A1 - Method of handling an object by a heavy lift crane, system and crane for use in said method

Info

Publication number: WO2023020936A1
Application number: PCT/EP2022/072574
Authority: WO
Inventors: Cornelis Martinus VAN VELUW
Original assignee: Itrec B.V.
Priority date: 2021-08-16
Filing date: 2022-08-11
Publication date: 2023-02-23
Also published as: NL2028975B1; EP4387916A1

Abstract

The invention relates to a method of handling an object by a crane, comprising prior to suspension of the object, tensioning the hoist cable to a first tension in an unwound stretch thereof that extends between the drums of the winches and one or more spooling and unwinding steps. In the latter the cable is simultaneously spooled onto one of the winches and unwound from the other, whilst the first tension is maintained in the unwound stretch, until at least one layer wound onto the drum of the one of the winches is tensioned to the first tension.

Description

METHOD OF HANDLING AN OBJECT BY A HEAVY LIFT CRANE, SYSTEM AND CRANE

FOR USE IN SAID METHOD

The present invention relates to the field of heavy lift cranes employing at least two winches for hoisting and lowering an object suspended from a travelling block of the crane. A heavy lift crane in the context of the present invention is taken to mean a crane capable of handling loads of at least 400 metric tonnes, e.g. of 1000 - 10000 tonnes.

Such heavy lift cranes are commonly applied offshore for installing large offshore structures such as a foundation of a wind turbine, for example a monopile. The crane employs a hoisting system comprising a hoist cable extending from one winch and multiple falls, e.g. 8 or more falls, between an upper block and a travelling block to another winch, via multiple guide sheaves. The hoist cable is commonly made of steel. An object, e.g. a wind turbine foundation, can be lifted and lowered by operating the winches. Lowering the object involves unwinding the cable from the drums of the winches, so as to increase the cable length of the cable falls between the upper and travelling block, and lifting the object involves spooling the cable onto the winches, so as to decrease the length of the falls.

The boom of the heavy lift crane is during the transfer generally in a horizontal rest position, supported by a boom rest, with the travelling block in a fully raised position, close to or against the upper block. Preferably, the travelling block is therein secured relative to the upper block so as to prevent swinging of the travelling block under the influence of wave-induced motion as the vessel sails to the installation site. Preferably for this purpose a block catcher is used, known from e.g. EP2133304 (figure 9 and paragraphs 0059-0064), from EP2088114 (catcher 111), and US2007098504 (figure 9).

Monopiles are often transferred on a vessel in a horizontal orientation to the installation site. This may be the same vessel as the one equipped with the heavy lift crane, yet may also be another vessel. Once arrived at the installation site, the boom is luffed upwards from the rest position to an operative position, thereby also lifting the travelling block still in its fully raised position - so that it ends up at the upper end of the hoisting range. Subsequently the travelling block of the heavy lift crane is lowered by operating the winches, unwinding the hoist cable from the drums of both winches, to subsequently engage one end of the monopile, e.g. by a pile gripper that has prior to the engagement been suspended underneath the travelling block. Thereafter, the hoisting cable is hauled in onto the drums of the winches to upend the monopile to a vertical orientation. The monopile is subsequently lowered in that vertical orientation onto the seabed by unwinding the hoist cable again from the drums of the winches of the crane. During upending, the other end of the monopile is supported, generally by an upending tool at the edge of the vessel.

Notably, during upending of the monopile, the hoist cable is not yet loaded by the full weight of the monopile. A substantially full support of the monopile follows after upending, during the subsequent lowering - thus after a considerable length of cable has already been spooled onto the winches under the tension of part of the weight of the monopile during upending.

A more recently introduced idea is to transfer the monopiles vertically to the installation site, so that upending thereof prior to the lowering is not necessary. In the installation process, once at the installation site, the heavy lift crane engages the already vertically oriented monopile after luffing of the boom with the travelling block in the fully raised position, subsequently displaces the monopile outboard the vessel, and thereafter lowers the monopile onto the seabed. In this process, the hoist cable is loaded by the full weight of the monopile directly after engagement thereof. The loading of the cable by the weight of the monopile generally takes place while the travelling block is still in a fully raised position close to or against the upper block - since the monopile is generally engaged by the upper end thereof, thus without any length of cable already having been spooled onto the winches under tension.

In the stretch of the hoist cable that is at the point of loading already wound around the drums of the winches, such abrupt increase in the load may lead to problems which may degrade or damage the hoist cable. This is caused by the fact that the stretch of the hoist cable that is unwound, and extends between the winches via the upper and the travelling block, is at once tensioned to a high extent by the heavy loading. This generally leads to one or more cable windings around the drum of each winch to be highly tensioned as well in an outer layer of these windings - whereas the cable windings in inner layers are only lightly, tensioned by the weight of the travelling block and optionally a tool suspended underneath it - e.g. a pile gripper. An adverse consequence is that the highly tensioned outer cable windings in the upper layer press heavily inwardly on the inner layers as the outer windings are caused by the abrupt high tension to be spooled considerably more tightly around the inner layers. This may lead to local material stresses in all layers, and to ‘cutting-in’ of the outer cable windings in between loosely spooled windings of the inner layers. Furthermore, the pressing, the cutting-in, and the high tensioning in general leading to stretching of the highly tensioned part of the cable, may cause the object to undesiredly move downwards slightly after suspension. Such problems may be worsened as the length of the cable, and thus, the number of layers of cable windings increases, as the instability of the layers may increase therewith. Often, the length of the cable wound around the drums of each winch is in the order of a kilometer, e.g. several kilometers, e.g. up to 10 kilometers.

It is an object of the present invention to alleviate these problems, or at least, to provide an alternative system for handling an object by a heavy lift crane.

The present invention provides a method according to claim 1.

The crane used in the method comprises a first winch having a first drum, and a second winch having a second drum. The crane may or may not have one or more additional winches. The hoist cable is envisaged to be made of steel.

The crane comprises an upper block, comprising one or more upper sheaves, and a travelling block, comprising one or more lower sheaves - as is known in the art. The travelling block comprises a load attachment device, for suspending the object from the crane. Such load attachment devices are known in the art and also commonly referred to as an object suspension device. The load attachment device is suitable for attaching the object to the travelling block - so as to suspend the object from the crane. A hoist cable of the crane extends from the first drum of the first winch, in one or more falls through the upper sheaves and the lower sheaves, to the second drum of the second winch, so that hauling the hoist cable onto the first and/or second winch, hoists the travelling block upwards towards the upper block, and unwinding the hoist cable from the first and/or second winch, lowers the travelling block, away from the upper block. The winches may be provided on the crane, inside a crane housing, or even below a deck of a vessel onto which the crane is provided, and the crane may as is common, comprise one or more guide sheaves along the boom for guiding the cable from the winches to the upper sheaves of the upper block.

At the start of the method, the cable is wound in multiple layers around at least the first drum. Preferably, the cable is also wound in multiple layers around the second drum, so that the weight and thickness of the cable windings is, advantageously, equally distributed over the winches. For example, the cable is wound in a similar or approximately the same amount of layers around both drums. Initially the cable has an initial, low tension, which emanates from the weight of the travelling block - generally in the order of 200 tonnes. In the initial state of the method, the travelling block is immobilized relative to the upper block.

The method comprises, firstly, operating the winches, such as to tension the hoist cable from the initial tension to a first, higher tension in the unwound stretch. This is envisaged to be accomplished by operating the winches such that these haul in a relatively small stretch of the cable onto the winches, thereby tightening the unwound stretch of the cable. Therein the immobilized travelling block relative to the upper block counteracts a length change of an unwound stretch of the cable that extends between the drums of the winches. An alternative option to immobilizing the travelling block relative to the upper block is, for example, to lock the winches while moving the upper block and the travelling block apart, so as to stretch out the cable thereby tightening the cable.

Inherently, the tightening of the unwound stretch may cause one or more windings in an outer layer of a part of the hoist cable still wound around the drum of each winch, to be tensioned to the first tension as well. The tensioning will generally penetrate in the cable windings up to the point at which friction at the interface between the windings has fully counteracted the extra stretching in the cable resulting from the tensioning.

The first tension is chosen such, that it is between the initial tension of the cable prior to the described tensioning, if any, and a second tension that will be caused in the cable by the weight of the object upon suspension of the object, after attachment thereof to the travelling block via the load attachment device. This weight is at least 400 tonnes, for example in the range of 1000 - 10000 tonnes.

In the method, the first tension is maintained in the unwound stretch - and as explained, possibly in one or more windings of the cables around the drums of the winches - while performing the following simultaneous spooling and unwinding. This maintaining of the first tension is also accomplished by operating the winches.

The first spooling step is formed by a simultaneous spooling of the hoist cable onto the second winch and unwinding of the hoist cable from the first winch. This is, thus, done whilst maintaining to apply the first tension in the unwound stretch of the hoist cable that is extending between the drums of the winches. Here, it is noted that the stretch of the hoist cable that is unwound and extends between the winches, inherently, changes as the cable is spooled. In particular, windings of the hoist cable around the first drum that were not tensioned by the tensioning step, may during this step become part of the unwound stretch as a result of the unwinding - so that in order to maintain tension in the unwound stretch, it may be necessary to apply traction to the unwound stretch during the spooling and unwinding. For example, this traction may be achieved by spooling the hoist cable onto the second winch at a slightly higher speed than unwinding the hoist cable from the first winch - or in an alternative method - by moving the upper block and travelling block further apart whilst spooling and unwinding. The simultaneous spooling and unwinding is done until at least one layer is wound around the second drum and is tensioned to the first tension, for example until at least one layer that underlies an outermost one of the multiple layers around the second drum is tensioned to the first tension.

The result of this step is that at least one layer of cable windings, e.g. resulting in one layer directly underneath the outer layer of cable windings, around the second drum is tensioned to the first tension, and that the unwound section also has the first tension. Thus, the second winch now has at least one layer which is spooled more tightly around the second drum than the layers initially on the drum, at the start of the method.

The method further comprises subsequently attaching the object to the load attachment device so as to suspend it from the crane, and the lifting and/or lowering of the object by operating the first and/or second winch. Upon suspension of the object, the weight thereof tensions the unwound stretch of the hoist cable extending between the winches, to a second, high tension that is higher than the first tension, for example around 1¹/₃ to 4 times higher, e.g. around 1 .5 times, around 2 times, or around 3 times as high.

For example, with the object having a mass of at least 1000 metric tonnes, it is envisaged that the first tension corresponds to a tension that would be present upon suspension of an object of at least around 250 tonnes, e.g. of at least around 300 tonnes, e.g. of at least around 500 tonnes.

Advantageously, because prior to the loading of the cable by the weight of the object, the second drum has in this method at least one layer which is relatively tightly spooled, the stretch of cable that now has the second tension is less likely to ‘cut-in’ the underlying layers, and local materials stresses in the wound cable may be lessened. After all the windings of the underlying layer can be moved less easily, since due to the first tension, these windings press harder inwardly, so that a higher surface friction is established at the interface with the layer - or drum surface - underneath it, to counteract any movement. This also applies to the laterally moving apart of cable windings of the underlying layer, so that the highly loaded cable can squeeze less easily between the underlying windings. The higher friction of the underlying layer with the still underlying layer or drum surface may also reduce the undesired effect of the object moving slightly downwards after suspension because of stretching of the cable - since this stretching may now be limited to a smaller part of the wound cable. In a preferred embodiment of the method, the spooling and unwinding is followed by a second spooling and unwinding, wherein the hoist cable is simultaneously spooled onto the first winch and unwound from the second winch, whilst maintaining to apply the first tension in the unwound stretch of the hoist cable extending between the winches. Alike in the first spooling and unwinding step, maintaining the first tension may involve applying traction to the unwound stretch. The simultaneous spooling and unwinding is done until at least one layer is wound around the first drum and is tensioned to the first tension, and e.g. at least one outermost layer remaining on the second drum has the first tension. Depending on the initial number of layers on the second drum at the start of the method, and the number of layers unwound during the second step, this at least one layer is for example the only layer still left on the second drum, or at least one layer underlying an outermost layer.

It is noted that herein, when the term 'step’ is used, this is not meant to indicate associated actions to necessarily be performed in a specific order.

In an embodiment, the second spooling an unwinding step involves unwinding only tensioned layers that have been spooled thereon in the first spooling and unwinding step, so that the at least one layer still left on the second drum after the second spooling and unwinding step was tensioned already in the first spooling and unwinding step. If at the start of the method the hoist cable was wound in one or more layers around the second drum, these loosely spooled layers remain in this example on the second drum after the second spooling and unwinding step, underneath the mentioned at least one layer with the first tension. The presence of one or more, e.g. underlying, tensioned layer(s) may already provide the mentioned advantages to a satisfactory extent for the cable wound on the second drum. In this case the second spooling and unwinding step basically results in a redistribution of the tensioned part of the cable over the winches. The second spooling and unwinding step may for example be done until the tensioned part of the cable is approximately equally distributed over the winches. Thus, until the first winch has a certain number of outermost layers that have the first tension, and the second winch has approximately the same number of outermost layers that have the first tension. This may be beneficial since after the object is suspended, both winches may be operated at the same time for lowering the object while the cable is on both winches unwound on underlying tightly spooled layers.

In another embodiment however, all of the layers that have been tensioned in the first spooling and unwinding step are unwound in the second spooling and unwinding step, and one or more, e.g. all, of the initially loosely spooled layers underneath these tensioned layers are tensioned during the second spooling and unwinding step. In this case the second spooling and unwinding step results in an additional part of the cable being tensioned, so that the mentioned possible advantages may be enhanced. For example, the second spooling and unwinding step may be followed by a distribution step wherein the - now extended - tensioned part of the cable is redistributed over the two winches - by spooling back a length of now tensioned cable onto the second winch again.

The mentioned advantages may be achieved also for the first winch, when the second spooling and unwinding step and/or a subsequent spooling back of tensioned cable onto the second winch results in at least one underlying layer having the first tension. Thus, in that case the first and the second winch each have at least one underlying layer which is spooled more tightly around the respective drum than initially, at the start of the method. Thus, the abovementioned possible advantages thereof as described above in relation to the second drum, now also apply to the first drum: less chance of cutting-in, disadvantageous local material stresses, and slight descent of the object after suspension.

In an embodiment, in the first spooling and unwinding step, the simultaneous spooling and unwinding is done until an innermost layer of the hoist cable initially wound around the first drum is tensioned to the first tension. This may involve unwinding the first drum until the innermost layer of windings there around is exposed, e.g. partly unwound as well such as to be tensioned. When the object is suspended and subsequently lifted and/or lowered directly after this first spooling and unwinding step, without performing the second spooling and unwinding step, this can e.g. be done by operating the second winch only, since it is the second winch which has the largest length of hoist cable wound on its drum, and has the tensioned layers. If the first spooling and unwinding step is, prior to the suspension of the object, however followed by the second spooling and unwinding step, as is preferred, this embodiment may have the advantage that after the spooling back of the cable onto the first winch during the second spooling and unwinding step, all layers of cable windings on the first winch are tensioned to the first tension, and thus, tightly spooled around the first drum. This may enhance the advantageous effects mentioned before - since these apply to all underlying layers wound on the second drum now.

In a preferred embodiment, the first and second spooling and unwinding step is followed by a step of distributing the tensioned part of the hoist cable over the winches by spooling back a length of the tensioned part of the cable onto the second winch. That is, by simultaneously spooling the cable onto the second winch and unwinding it from the first winch, for example until both winches have a similar or approximately equal amount of tensioned layers wound around their respective drums. This may be advantageous for lifting and lowering of the object, since this would facilitate to, as is common and preferred, operate both of the winches for performing the lifting and lowering along the height range of the travelling block.

For example, as is preferred in the use of heavy lift cranes, the winches have an approximately equal total amount of layers wound around their respective drums prior to the suspension of the object. This is favorable in terms of the distribution of the weight and total thickness of the layers of the cable windings together around the drum.

In an embodiment, in the second spooling and unwinding step, the simultaneous spooling and unwinding is done until an innermost layer of the hoist cable initially wound around the second drum is tensioned to the first tension. This may involve unwinding the second drum until the innermost layer of windings there around is exposed, e.g. partly unwound as well, such as to be tensioned. The object may be suspended at this point, e.g. operating only the first winch for subsequently lowering and/or lifting the object, however it is preferred that the second step is followed by the mentioned distribution step. In that case this embodiment results in all layers of the second drum being tensioned to the first tension, and thus tightly spooled around the second winch - which may provide enhanced advantageous effects of the method. On the other hand, other embodiments wherein only a part of the layers initially wound around the second drum are tensioned to the first tension, may in certain applications however be advantageous as the capacity of the first drum can be limited such as to enable accommodating only a part of the layers spooled thereon from the second drum. In embodiments where substantially all or a large part of layers are to be tensioned to the first tension, the capacity of the first drum should be large enough to allow spooling of the cable length of these layers thereon.

In an embodiment, the spooling and unwinding step(s) is or are performed such as to tension the cable along substantially its whole length.

In an embodiment, initially, thus at the start of the method, the hoist cable is wound on the drums of both winches in multiple layers, for example in a similar or approximately equal amount of layers. In this case, both the first and the second spooling and unwinding step are necessary to tension the cable over substantially its entire length.

In another embodiment, initially, the cable is wound in multiple layers around the first drum, and in at most one layer around the second drum, and the first spooling and unwinding step is performed until the innermost one of the layers initially wound around the first drum is tensioned to the first tension. In this embodiment the cable can be tensioned to the first tension in one single spooling and unwinding step.

The tensioning step and/or the spooling and unwinding step(s) and/or the spooling back step, if present, are in the method preferably performed whilst maintaining a substantially constant length of the unwound stretch of the cable by the immobilization of the travelling block relative to the upper block. This immobilization establishes keeping a relative positioning of the drums and the sheaves via which the cable extends, substantially constant.

It is preferred that at least the method steps of tensioning and the spooling and unwinding steps are performed with the travelling block in a fully raised position, thus against or close to the upper block, so that the length of the cable falls between the upper and lower sheaves is reduced to a minimum. Preferably also any spooling back step, if present, is performed with the travelling block in the fully raised position. Preferably, the travelling block is in the fully raised position continuously from the start of the method until directly prior to the suspension of the object - for example also during suspension of the object, e.g. a vertically oriented monopile.

In an embodiment, a length of the unwound stretch is maintained during these same method steps. In an embodiment, a position of the upper sheaves relative to the lower sheaves is maintained during these method steps. In an example this is achieved by releasably securing the travelling block relative to the upper block during this time, for example by securing the travelling block to the upper block. In an embodiment, such securing of the travelling block to the upper block is established by means of the mentioned block catcher.. This enables the tensioning to be achieved by having the winches haul in the cable slightly - this would lead to stretching and tensioning of the unwound stretch.

An alternative method is envisaged wherein the tensioning of the unwound stretch of the hoist cable to the first tension is achieved by moving the upper and lower sheaves apart while the winches are blocked. In particular with a block catcher, this may be achieved by extending the pin of the block catcher, for example telescopically.

It is noted that in order to achieve the mentioned advantages, the first tension does not necessarily have to be one specific amount that is continuous along the tensioned part of the cable. The first tension may for example also vary slightly, for example by up to 20%, e.g. up to 10%, e.g. up to 5%, from an average first tension, within a range that lies between zero and the second tension. To have a significant advantageous effect, it is envisaged that the average first tension is 25% to 75% of the second tension, for example 30-70%, for example 40-60%.

In an embodiment, the method further comprises, directly prior to suspension of the object, thus at least after the spooling and unwinding step(s) and any spooling back step, a step of at least partly releasing the first tension in the unwound stretch of the cable. Notably, this is done while maintaining the first tension in the tensioned underlying layers wound on the first and second drum. This may advantageously lead to a lower second tension, and therefore, a smaller difference between the first and second tension. In an example, this is done by a slight unwinding of the hoist cable from the winches.

In an embodiment, the first spooling and unwinding step is preceded by a lifting of the travelling block from a lower position to the raised position thereof, e.g. with a very light load attached to the load attachment device. This is done by operating the winches such as to spool the cable simultaneously on both drums in multiple layers, e.g. with a substantially even distribution, and with a favorable, orderly arrangement of the windings. This may result in a favorable starting point for the first spooling and unwinding step.

In an embodiment the first tension corresponds to a tension which would be caused by an object with a mass of at least 300 tonnes, e.g. at least 500 tonnes, being suspended from the crane via the load attachment device.

In an embodiment the winches are operated based on sensor measurements of the cable tension. As is known in the art, such sensors may be provided in the winches. In the step of tensioning, the winches may be operated until a desired first tension is reached - for example until the first tension is around 25%-75% of the second tension.

In an embodiment the method is performed on a vessel.

In an embodiment the object is an offshore structure.

In an embodiment the object has a weight of at least 1000 mt, e.g. of 1500 - 2500 mt, e.g. the object is an offshore structure with a weight of at least 1000 mt, e.g. of 1500 - 2500 mt.

In an embodiment the object is a foundation for an offshore wind turbine, e.g. a monopile configured to serve as foundation of an offshore wind turbine. In an embodiment wherein the object is a foundation of a wind turbine, e.g. a monopile, the load attachment device is a lifting tool, e.g. a gripper for engaging an upper end of the foundation, as known from e.g. WO2018139918, and the step of attaching the foundation involves engaging by means of the gripper the upper end of the foundation.

In an embodiment wherein the object is a foundation of a wind turbine, e.g. a monopile, the step of lifting and/or lowering of the object includes lowering the foundation through the splash zone and onto the seabed, e.g. partly into the seabed.

The invention furthermore relates to a crane system according to claim 15, which is particularly suitable for use in the method according to the invention. The crane system comprises a heavy lift crane as described, and a control unit. The control unit is operatively connected to the first and second winch of the crane, and is programmed to operate these winches such as to perform the method steps of the method according to the invention - i.e. at least the tensioning step and the first spooling and unwinding step.

The crane of the crane system may be provided with one or more sensors for measuring a tension in the cable, wherein the sensors are operatively connected to the control unit, and the control unit is programmed to operate the winches, based on measurements of the cable tension by the sensors, e.g. the sensors being provided in the winches, wherein e.g. in the step of tensioning, the winches are operated until a desired first tension is reached, e.g. until the first tension is around 25%-75% of the second tension.

The crane system may further comprise means to immobilize the travelling block relative to the upper block, for the purpose of keeping a relative positioning of the drums and the sheaves via which the cable extends, substantially constant, so as to counteract a length change of the unwound stretch being tensioned to the first tension. Such means may for example be configured to releasably secure the travelling block relative to the upper block, so that the lower sheaves do not move upwards as a consequence of the increased tension in the cables. The means are, preferably, configured to counteract an upwards force on the travelling block by the cable of corresponding to at least around 3 MN, e.g. of at least 5 MN. The means may for instance include a cable secured to a stationary surface and to the travelling block and/or be in the form of one or more rigid elements for releasably securing the travelling block to the upper block, configured to withstand a compressive stress of at least around 3 MN, e.g. of at least 5 MN. In an embodiment the one or more rigid elements are in the form of a block catcher, releasably securing the blocks to one another, and adapted to withstand said compressive stress. In an embodiment, the control unit is furthermore programmed to keep the travelling block secured to the upper block during the tensioning step and/or the spooling and unwinding steps and/or the spooling back step, if present. This is, of course, also possible for other means for maintaining the length of the unwound stretch.

The invention furthermore relates to a heavy lift crane which is particularly suitable for use in the method according to the invention, for example as a part of the crane system according to the invention. The heavy lift crane is, as described in relation to the method, configured for handling an object of at least 400 tonnes and comprises the first and second winch, the upper block and travelling block, and hoist cable as described. The crane according to the invention further comprises means immobilizing the travelling block relative to the upper block, for the purpose of keeping a relative positioning of the drums and the sheaves via which the cable extends, substantially constant, so as to counteract a length change of the unwound stretch being tensioned to the first tension. As described in relation to the system, such means may e.g. be in the form of one or more rigid elements for releasably securing the travelling block to the upper block, for example configured to withstand a compressive stress of at least around 3 MN, e.g. of at least around 5 MN. In an embodiment such rigid elements may be in the form of the described block catcher, securing the blocks to one another, and adapted to withstand said compressive stress.

The invention furthermore relates to a vessel provided with a crane system according to the invention and/or a crane according to the invention.

The invention will now be described with reference to the appended figures. In the figures: figure 1 illustrates on a vessel, a crane usable in a method according to the invention, figure 2 illustrates schematically a hoisting assembly of the same crane, figures 3-4 illustrate a situation in winches in a method according to the prior art, figures 5a-b illustrate schematically a method according to the prior art, figures 6a-b illustrate schematically a tensioning step of a method according to the invention, figures 7a-b illustrate schematically a first spooling and unwinding step of the same method, figures 8a-b illustrate schematically a second spooling and unwinding step of the same method, figures 9a-b illustrate schematically a spooling back step of the same method, figures 10a-b illustrate schematically a step of suspending the object in the same method. Figure 1 shows a crane 7 suitable for use in the method according to the invention. The crane 7 comprises a first winch 1 , a second winch 2, an upper block 3, a travelling block 4, and a hoist cable 5 which extends from the first winch 1 to the second winch 2. The crane 7 is provided on a vessel 6. In figure 1 the travelling block 4 is shown in a lower position thereof.

Figure 2 shows schematically a hoisting assembly of the crane 7 in isolation from the rest of the crane 7. It is illustrated that the first winch 1 has a first drum 11 , and the second winch 2 has a second drum 21. The upper block 3 comprises multiple upper sheaves 32. The travelling block 4 comprises one or more lower sheaves 42 and a load attachment device 41 , for suspending an object from the crane 7. Here, the load attachment device 41 is illustrated as a hook. The hoist cable 5 extends from the first drum 11 of the first winch 1 , in multiple falls through the upper sheaves 32 and the lower sheaves 42, to the second drum 21 of the second winch 2, so that hauling the hoist cable 5 onto the first and/or second winch 1 ,2 hoists the travelling block 4 upwards towards the upper block 3, and unwinding the hoist cable 5 from the first and/or second winch 1 ,2 lowers the travelling block 4, away from the upper block 3.

The cable 5 has an unwound stretch 5s, which is formed by the part of the cable 5 that is not wound on either of the drums 11,21, and thus extends between the drums 11,21. When referring to ‘the unwound stretch’ during the method, this is intended to mean the stretch of the cable that is at that point in time the part of the cable 5 that is unwound and extending between the drums 11 ,21. The cable 5 is guided from the first winch 1 to the sheaves 32 of the upper block 3 via first guide sheaves 12. The unwound stretch 5s of the cable 5 is guided from the second winch 2 to the sheaves 32 of the upper block 3 via second guide sheaves 22.

The part of the cable 5 that is actually wound around the first drum 11 of the first winch 1 is indicated by 5w1. The part of the cable 5 that is actually wound around the second drum 21 of the second winch 2 is indicated by 5w2.

Figures 3 and 4 illustrate, schematically, a front view of the first winch 1. The circles represent cross-sections of the windings of the cable 5 at the top and bottom of the drum 11. The slanted longitudinal lines indicated by 5w1 represent the longitudinal extension of the cable 4 around the front half of the drum 11 from the top to the bottom of the drum 11. Thereby it is schematically shown that the hoist cable 5 is wound around the winch 11 in multiple windings which form multiple layers 5w1.1, 5w1.2, and 5w1.3. Thereof the layer 5w1.1 is the innermost layer, the layer 5w1.3 is the outermost layer, and the layer 5w1.2 is the layer which underlies the outermost layer 5w1.3. By the vertical shading, a package of windings is indicated. When the cable is loosely spooled, and the unwound stretch 5s of the cable 5 is loaded, the cable winding 5w1.3L in the outer layer from which the unwound stretch 5s departs, is highly tensioned. This means that this winding presses inwardly on the windings of the package in the layers 5w1.2 and 5w1.3 underlying the loaded cable winding 5W1 ,3L. When the cable 5 is loosely spooled around the drum 11 , this may result in an unstable package: the loaded cable winding 5w1.3L may squeeze between the underlying cable windings of the package, because the windings around the package move apart under the pressure of the loaded winding 5w1.3L. This unstable package is shown in figure 4. The movability of the windings around the package results from the large difference in tension between the loaded winding 5w1.3L - these are more likely to move since surface friction between the loosely spooled windings is lower. It can be seen that this in this example leads to ‘cutting-in’ of the loaded winding in the underlying layers 5w1.1, 5w1.2 - resulting in unfavorable mechanical load profiles of the wound part 5w1 of the cable 5 and the earlier mentioned slight lowering of an object after suspension.

The present invention is aimed at reducing the difference in cable tension between the windings of a hoist cable around the drum of a winch in such a hoisting assembly, so as to alleviate these problems.

Figures 5a-b illustrate a prior art method. Figure 5a shows in the same schematic view the same hoisting assembly of the crane 7 as in figure 2 while being loaded by an object with a weight F₂. Figure 5b shows both winches 1 and 2, in the same view as figures 3 and 4. It is noted, that whereas figures 3 and 4 both show only the first winch 1 in two different points in time, figure 5b shows both the first winch 1 and the second winch 2 of the hoisting assembly in the situation of figure 5a.

In the prior art method, the cable 5 is initially loosely spooled around the drums 11 ,21. The windings have zero or only a very low tension To. When the object is attached to the load attachment device, the weight F₂ is exerted on the travelling block and counteracted by the tension T₂ in the unwound stretch 5s of the cable 5 in the falls through the upper and lower sheaves 32,42. This tension is continued in the unwound stretch 5s of the cable 5 between the upper sheaves and the drums 11 ,21. The tension T₂ further penetrates in the cable 5 in the windings of each outermost layer 5w1.3, 5w2.3 around the respective drums 11 ,21 , until friction between the windings has counteracted the tension T₂. As a result, these windings of the outermost layers 5w1.3, 5w2.3 have the high tension T₂ - a large difference with the zero or low tension To in the underlying layers 5w1.1, 5w1.2, 5w2.1, 5w2.2. This may lead to one or more of the aforementioned problems. It is noted that in the figures, the number of windings and layers, and the relative dimensions of the cable, drums, etc. are not true to nature and are chosen as such only to illustrate the method in a clear and orderly way. For example, the number of windings and layers is in practice commonly much larger - given the usual length of the cables in the order of a kilometer, e.g. several kilometers. Also, for example the tensioning of the unwound stretch 5s may in practice not penetrate into the cable throughout a complete outermost layer of windings. Alike the initial tension To and the first tension Ti, the tension T₂ may also be a range instead of one continuous value.

Figures 6a-b to 10a-b illustrate an embodiment of a method according to the invention.

Initially, the hoist cable 5 is wound in multiple layers 5w1.1 , 5w1.2, 5w1.3 around at least the first drum 11 , and in multiple layers 5w2.1, 5w2.2, 5w2.3 around the second drum 21 . Initially the cable 5 has in all of these layers 5w1.1, 5w1.2, 5w1.3, 5w2.1 , 5w2.2, 5w2.3 the zero or low tension To - and is thus loosely spooled around the drums 11 ,21.

Figures 6a-b illustrate tensioning the hoist cable 5 to a first tension Ti in an unwound stretch 5s thereof that extends between the drums 11 ,21 of the winches 1 ,2. This is done by hauling in a small stretch of the cable 5s onto the winches 1 ,2 so as to tighten the unwound stretch 5s of the cable 5. This hauling in is illustrated in figure 6a by the curved arrows around the drums 11,21 of the winches 1 ,2 showing the direction of the rotation of the drums 11,21 establishing such hauling in. Therein, the length of the unwound stretch 5s is kept constant, by keeping the relative positioning of the drums 11 ,21 and sheaves 12,22,32,42 constant. For the upper and lower sheaves 32,42, this is accomplished by securing prior to the tensioning, the travelling block 4 to the upper block 3, by means of a block catcher. Here, the block catcher is represented very schematically as comprising a pin 43 provided on the travelling block 4 being geometrically locked inside a mating, cylindrically shaped receiver 33 - from the prior art possible shapes are known. Thus, the cable 5 stretches slightly in the unwound stretch 5s. Figure 6b shows the result of the tensioning: the outer layers 5w1.3 and 5w2.3 of windings are tensioned to the tension Ti. The underlying layers 5w1.1 , 5w1.2, 5w2.1 and 5w2.2 still have the low or zero tension To - are thus still loosely spooled around the drums 11,21.

Figures 7a-b illustrate a first spooling and unwinding step of the method. In this step the hoist cable 5 is simultaneously spooled onto the second winch 2 and unwound from the first winch 1 , see the curved arrows for the rotation directions. This is done whilst maintaining the first tension Ti in the unwound stretch 5s of the hoist cable 5 extending between the drums 11 ,21. This is done until multiple layers 5w2.3, 5w2.4 underlying the now outermost one 5w2.5 of the multiple layers 5w2.1 , 5w2.2, 5w2.3, 5w2.4, 5w2.5 now wound around the second drum 21 are tensioned to the first tension Ti - as shown in figure 7b. The spooling and unwinding step entails that the initially loosely spooled layer 5w1.2 which was on the first drum 11 in the tensioning step, see figure 6b, is tensioned, as it became part of the unwound stretch 5s as a result of the unwinding from the first drum 11. This is achieved by the spooling around the second drum 21 taking place at a slightly higher speed than the unwinding from the first drum 11. The result is illustrated in figure 7b. Furthermore, the initially loosely spooled innermost layer 5w1.1 is tensioned by the spooling and unwinding step: the tensioning of the unwound stretch 5s has penetrated in this layer 5w1.1.

In principle, when the object with the weight F2 would be suspended now after this first spooling and unwinding, the situation may already be improved for the part of the cable 5w2 wound around the second drum 21. After all, this would result in the outer layer 5w2.5 to be tensioned to T2, which would press on the underlying layers 5w2.3 and 5w2.4. For these layers, a more favorable loading - and less cutting in, smaller descent of the object after suspension - may already result from the tension T1 in the windings of these layers. An improvement would still be present if the object is lowered by unwinding the outer layer 5w2.5 from the second drum 21 , since there is still a tensioned layer 5w2.3 underlying the then outermost layer 5w2.4 which would then be tensioned to the tension T2.

Figures 8a-b illustrate a second spooling and unwinding step of the method. In this step the hoist cable 5 is simultaneously spooled onto the first winch 1 and unwound from the second winch 2, see the curved arrows for the rotation directions. This is done whilst maintaining the first tension T1 in the unwound stretch 5s of the hoist cable 5 extending between the winches 1 ,2, until the layers 5w1.1 , 5w1.2, 5w1.3, 5w1 .4 that underly outermost layer 5w1.5 of the multiple layers that are now wound around the first drum 11 are tensioned to the first tension, and one outermost layer 5w2.1 of cable windings around the second drum 21 has the first tension - namely, the only layer 5w2.1 still left on the second drum.

At this point, the cable 5 is tensioned to the first tension T1 along substantially its entire length. This is the consequence of both the first and second spooling and unwinding step being performed respectively until the innermost one 5w1.1, 5w2.1 of the layers initially wound around the first and second drum 11 ,21 is tensioned to the first tension T1.

Figures 9a-b illustrate a redistribution of the cable over the winches 1 ,2 - so that these end up with substantially the same number of layers 5w1.1, 5w1.2, 5w1.3, 5w2.1 , 5w2.2, 5w2.3. This is done by spooling back a tensioned part of the hoist cable 5 onto the second winch 2, while unwinding the first winch 1 - see the curved arrows - and maintaining the first tension in the unwound stretch 5s.

Figures 10a-b illustrate a step of suspending the object from the crane 7 - or actually, the result thereof. The suspension is done by attaching the object to the load attachment device 41. Upon the suspension of the object, the weight F₂ thereof tensions the unwound stretch 5s of the hoist cable 5 extending between the winches 1 ,2 to a second tension T₂ that is higher than the first tension Ti, for example around 1/3 to 3 times higher. The higher tension T₂ is illustrated by a more bold shading in the cable cross-sections. The higher tension T₂ penetrates through the cable in the outermost layers 5w1.3 and 5w2.3 around the drums 11 ,21. Because of the first tension Ti in the underlying layers 5w1.1 , 5w1.2, 5w2.1, 5w2.2 cutting-in by the windings of the outermost layers 5w1 .3 and 5w2.3 in the underlying layers 5w1.1, 5w1.2, 5w2.1, 5w2.2 may be reduced or diminished, evenas unfavorable local material stresses. On top of that, the tighter spooling of the underlying layers 5w1.1 , 5w1.2, 5w2.1, 5w2.2 may result in an increased capacity of the winches 1 ,2 - thus a larger amount of cable 5 to be spooled thereon - as stacking of the windings is more efficient and compact than when loosely spooled.

After the suspension step, the object may be lifted and/or lowered by operating the first and/or second winch 1 ,2.

It is envisaged that during the spooling and unwinding steps, the distribution step, and the suspension, the length of the unwound stretch 5s is still kept constant by keeping the relative positioning of the drums 11 ,21 and sheaves 12,22,32,42 constant. This includes leaving the travelling block 4 secured to the upper block 3 during at least this time.

It is envisaged that prior to the suspension of the object, the first tension T 1 may be released in the unwound stretch 5s, so that the second tension T₂ does not include the first tension Ti and the difference between Ti and T₂ is further reduced.

The suspension may be done while the travelling block is still in a raised position thereof - which is for example envisaged when the object is a monopile stored in a vertical orientation on the vessel. In this case, the method comprises after the suspension, moving the monopile outboard, e.g. by slewing the crane around a vertical slew axis thereof, and lowering of the monopile through the splash zone and towards the seabed for installation thereof by unwinding the cable 5 from the winches 1,2.

Claims

C L A I M S

1. Method of handling an object by a heavy lift crane (7), the object having a mass of at least 400, e.g. at least 1000 metric tonnes, the crane comprising: a first winch (1) having a first drum (11), a second winch (2) having a second drum (21), an upper block (3) comprising upper sheaves (32), a travelling block (4) comprising lower sheaves (42) and a load attachment device (41) for suspending the object from the crane, and a hoist cable (5) which extends from the first drum (11) of the first winch (1), in multiple falls through the upper sheaves (32) and the lower sheaves (42), to the second drum (21) of the second winch (2), wherein in an initial state, the hoist cable (5) is wound in multiple layers (5w1.1 , 5w1.2, 5w1.3) around the first drum (11) and, preferably, also in multiple layers (5w2.1 , 5w2.2, 5w2.3) around the second drum (21), and has an initial, low tension (To), and wherein, in the initial state, the travelling block (4) is immobilized relative to the upper block (3), the method comprising, starting from said initial state, operating the winches (1 ,2) so as to:

- tension the hoist cable (5) from the initial tension (To) to a higher first, intermediate tension (Ti) in an unwound stretch (5s) thereof that extends between the drums (11,21) of the winches (1 ,2), and simultaneously spooling a length of the hoist cable (5) onto the second winch (2) and unwinding the hoist cable (5) from the first winch (1) whilst maintaining the first tension (Ti) in the unwound stretch (5s) of the hoist cable (5), until at least one layer (5w2.3, 5w2.4) is wound around the second drum (21) and is tensioned to the first tension (Ti), the method further comprising: attaching the object to the load attachment device (41) so as to suspend the object from the crane (7), lifting and/or lowering the object by operating the first and/or second winch (1 ,2), wherein upon suspension of the object, a weight (F₂) thereof tensions the unwound stretch (5s) of the hoist cable (5) extending between the winches (1 ,2) to a second, high tension (T₂) that is higher than the first tension (Ti), for example around 1¹/₃ to 4 times higher.

2. Method according to claim 1, wherein the spooling and unwinding is done until an innermost one (5w1.1) of the layers initially wound around the first drum (11) is tensioned to the first tension (Ti).

3. Method according to claim 1 or 2, further comprising, after the spooling onto the second winch (2) and unwinding from the first winch (1), and prior to suspending the object: simultaneously spooling the hoist cable (5) onto the first winch (1) and unwinding the hoist cable (5) from the second winch (2), whilst maintaining the first tension (Ti) in the unwound stretch (5s) of the hoist cable (5) extending between the winches (1 ,2), until at least one layer (5w1.1, 5w1.2, 5w1.3, 5w1.4) is wound around the first drum (11) and is tensioned to the first tension, and at least one layer (5w2.1) of cable windings remaining around the second drum (21) has the first tension (Ti).

4. Method according to claim 3, wherein the spooling onto the first winch (1) and unwinding from the second winch (2) is done until an innermost one (5w2.1) of the layers initially wound around the second drum (21) is tensioned to the first tension (Ti).

5. Method according to at least claim 2, wherein the spooling and unwinding is performed such as to tension the hoist cable (5) to the first tension (Ti) along a majority of its length, e.g. along substantially its entire length.

6. Method according to any one or more of the preceding claims, wherein after at least the spooling the hoist cable (5) onto the second winch (2) the method comprises: spooling back a tensioned part of the hoist cable (5) onto the second winch (2), while unwinding the first winch (1) and maintaining the first tension in the unwound stretch (5s), so as to distribute the hoist cable (5) over both drums (11,21), at least in multiple layers (5w1.1 , 5w1.2, 5w1.3) around the first drum (11) and in multiple layers (5w2.1, 5w2.2, 5w2.3) around the second drum (21).

7. Method according to any one or more of the preceding claims, wherein in the initial state, the travelling block (4) is immobilized relative to the upper block (3) as the travelling block (4) is releasably secured to the upper block (3).

8. Method according to any one or more of the preceding claims, wherein the first tension (Ti) corresponds to a tension which would be caused by an object with a mass of at least 300 tonnes, e.g. at least 500 tonnes, being suspended from the crane (7) via the load attachment device (41).

9. Method according to any one or more of the preceding claims, wherein the winches (1 ,2) are operated for spooling and unwinding based on measurements of actual cable tension, e.g. one or more sensors being provided in the winches (1 ,2), wherein in the tensioning, the winches are operated until a desired magnitude for the first tension (Ti) is reached, e.g. until the first tension is around 25-75% of the second tension (T₂).

10. Method according to any one or more of the preceding claims, wherein the crane (7) is provided on a vessel (6) and the method is performed on the vessel (6).

11 . Method according to claim 10, wherein the object is a monopile, and the method is performed on the vessel (6) at an offshore installation site for the monopile, wherein the load attachment device (41) is a lifting tool, e.g. a gripper, for engaging an upper end of the monopile, and the attaching of the object, i.e. the monopile, involves engaging by means of the lifting tool the upper end of the monopile, and wherein the lifting and/or lowering of the object, i.e. the monopile, includes lowering the monopile through the splash zone and onto the seabed, e.g. partly into the seabed.

12. Crane system of a heavy lift crane (7) and a control unit, e.g. for use in the method according to any one or more of claims 1 - 11 , the crane being configured for handling an object of at least 400 tonnes, e.g. at least 1000 tonnes, and comprising: a first winch (1) having a first drum (11), a second winch (2) having a second drum (21), an upper block (3) comprising upper sheaves (32), a travelling block (4) comprising lower sheaves (42) and a load attachment device (41), for suspending the object from the crane, and a hoist cable (5) which extends from the first drum (11) of the first winch (1), in one or more falls through the upper sheaves (32) and the lower sheaves (42), to the second drum (21) of the second winch (2), the control unit being operatively connected to the winches (1,2) and programmed to operate the winches (1 ,2) such as to perform the following method: -21-

- tensioning of the hoist cable from an initial tension (To) to a higher first, intermediate tension (Ti) in an unwound stretch (5s) thereof that extends between the drums (11,21) of the first and second winches (1 ,2), and simultaneously, spooling of the hoist cable (5) onto the second winch (2) and unwinding of the hoist cable (5) from the first winch (1) whilst maintaining the first tension (Ti) in the unwound stretch (5s) of the hoist cable (5) extending between the drums (11,21), until at least one layer (5w2.3, 5w2.4) is wound around the second drum (21) and is tensioned to the first tension (Ti), wherein the first tension (Ti) is intermediate the initial tension (To) and a second, high tension (T2) caused in the unwound stretch (5s) of the hoist cable (5) extending between the drums (11,21) by a predetermined weight (F2) of an object of at least 400 tonnes to be suspended from the crane, upon suspension thereof from the crane (7) via the load attachment device (41), for example the first tension (Ti) being around 25-75% of the second tension (T2).

13. Crane system according to claim 12, wherein the control unit is furthermore programmed to operate the winches (1,2) such as to perform the method according to any one or more of claims 2 - 6.

14. Crane system according to claim 12 or 13, wherein the crane (7) is provided with one or more sensors for measuring a tension in the hoist cable, wherein the sensors are operatively connected to the control unit, and the control unit is programmed to operate the winches (1,2) based on measurements of the cable tension by the sensors, e.g. the sensors being provided in the winches (1,2), wherein e.g. in the tensioning, the winches are operated until a desired magnitude of the first tension (Ti) is reached, e.g. until the first tension is around 25-75% of the second tension (T₂).

15. Crane system according to any one or more of claims 12 - 14, further comprising means to immobilize the travelling block (4) relative to the upper block (3), e.g. the immobilizing means being configured to counteract an upwards force on the travelling block (4) by the hoist cable of corresponding to at least around 3 MN, e.g. at least around 5 MN.

16. Crane system according to claim 15, wherein the means for immobilizing the travelling block (4) relative to the upper block (3) are in the form of one or more rigid elements, e.g. arranged on both of the travelling block (4) and the upper block (3), the rigid elements being configured for releasably securing the travelling block (4) to the upper block (3), e.g. -22- configured to withstand a compressive stress of at least around 3 MN, e.g. of at least around 5 MN, e.g. wherein the control unit is furthermore programmed to keep the travelling block (4) secured to the upper block (3) during the tensioning and/or the spooling and unwinding and/or the spooling back, if present.

17. Heavy lift crane (7) for use in the method of any one or more of claims 1 - 11 , the crane being configured for handling an object of at least 400 tonnes, and comprising: a first winch (1) having a first drum (11), a second winch (2) having a second drum (21), an upper block (3) comprising upper sheaves (32), a travelling block (4) comprising lower sheaves (42) and a load attachment device (41) for suspending the object from the crane, and a hoist cable (5) which extends from the first drum (11) of the first winch (1), in one or more falls through the upper sheaves (32) and the lower sheaves (42), to the second drum (21) of the second winch (2), and the crane further comprises means to immobilize the travelling block (4) relative to the upper block (3), for example the immobilizing means being configured to counteract an upwards force on the travelling block (4) by the hoist cable (5) of corresponding to at least around 3 MN, e.g. of at least around 5 MN.

18. Crane according to claim 17, wherein the immobilizing means are in the form of one or more rigid elements, e.g. arranged on the travelling block (4) and the upper block (3), and wherein the rigid elements are configured for releasably securing the travelling block (4) to the upper block (3), e.g. configured to withstand a compressive stress of at least around 3 MN, e.g. of at least around 5 MN.

19. Vessel (6) provided with a crane system according to any one or more of claims 12- 16, and/or with a crane according to claim 17 or 18.