Crane load stabilization using a tugger line

Abstract

Title: Crane load stabilization using a tugger line Abstract Tugger line system for stabilization of a load suspended in an offshore crane system, comprising: a tugger line to be engaged with the load; a winch fixed or fixable in the offshore crane system and configured to reel the tugger line in or out from the winch so as to vary a working length of the tugger line; and a damper assembly engaged or engageable with the tugger line between the winch and the load, wherein the damper assembly, when engaged, is configured to passively attenuate fluctuations of tension in the tugger line, said fluctuations being induced by movement of the load with respect to the winch. [Fig. 3]

Description

P 137407NLOO Title: Crane load stabilization using a tugger line FIELD The invention relates to a tugger line system for stabilization ofa load suspended in an offshore crane system, as well as to: a combination of the tugger line system and the offshore crane system; an offshore vessel or structure comprising the combination; uses; a method of stabilizing a load suspended in an offshore crane system; a damper assembly for the tugger line system; and a method ofproviding the tugger line system. BACKGROUND Offshore installation ofoffshore wind turbines can be challenging, in particular due to size and weight ofthe monopile and / or other components, sea and weather conditions, and a desire for cost efficiency.A particular challenge concerns stabilization ofthe monopile suspended in an offshore crane system, e.g. on a vessel at a desired offshore installation location. Similar considerations apply to other types ofloads in offshore crane systems. More stable loads are easier and safer to handle and put less strain on the crane system. At the same time, it is generally desired to move loads to their desired positions relatively quickly and to limit weight and complexity ofthe crane system. For load stabilization during crane operations, it is known to use tugger lines engaged with the load, wherein a winch can be controlled to reel the tugger line in or out from the winch so as to vary a working length ofthe tugger line. Essentially, amaximum distance between the winch and the load can be imposed in this way. However, it has been found that loads may still sway in response to disturbances. In view thereof, further improvement is desired. SUMMARY An object is to improve load stability during offshore crane operations. An object is to facilitate relatively effective and efficient installation of offshore wind turbines. An aspect provides a tugger line system for stabilization of a load suspended in an offshore crane system. The tugger line system comprises: a tugger line to be engaged with the load; and a winch fixed or fixable in the offshore crane system, in particular to a load bearing part thereof, and congured to reel the tugger line in or out from the winch so as to vary a working length ofthe tugger line. The tugger line system comprises a damper assembly engaged or engageable with the tugger line between the winch and the load, wherein the damper assembly, when engaged, is congured to passively attenuate uctuations oftension in the tugger line. Said uctuations are in particular induced by movement ofthe load with respect to the winch. Advantageously, such a damper assembly can reduce swinging of the load, in particular in combination with the functionality ofthe winch. Surprisingly, while the passive attenuation oftension uctuation may be considered to somewhat attenuate the traditional position control from the winch, it has been found that overall load stability tends to improve. In this respect, it is considered that in a known tugger line system without the damper assembly, the loadmay swing towards the winch by some transient disturbance, after which the loadmay swingback away from the winch until reaching amaximum distance imposedby the winch. At that time, the loadmay stop and reverse, resulting in cyclic or oscillatory swinging and thus ongoing load instability. By contrast, by passively attenuating uctuations oftension using the damper assembly, peak tensions can effectively be reduced, and thereby the oscillatory swinging can be reduced. It is considered that the damper assembly can dissipate energy from such swinging motions, thereby promoting stability. Meanwhile, the traditional functionality ofthe combination ofthe tugger line and the winch can essentially be maintained. In other words, the addition ofthe damper assembly need not interfere with the normal operation ofthe tugger line. As the damper assembly is congured to perform the attenuation passively, control ofthe tugger line need not be complicated by the addition ofthe damper assembly. In the present context, the passive attenuation ofuctuations of tension in the tugger line can be regarded as a form ofpassive damping, i.e. damping that does not require active control or an external power source. Nevertheless, it shall be appreciated that in some embodiments an external power source or active controlmay be associated with the damper assembly. For example, the damper assemblymay be configured to be activated or deactivated or otherwise adjusted using active control, or an external power source may be used to prepare the damper assembly for use. During use, energy for the passive attenuation is essentially extracted from the process to be attenuated, e.g. in the present case swinging ofthe load. In contrast, active attenuation or damping involves active control and an external power source, wherein for example in response to a sensed disturbance a controller is congured to activate generation of a counter force using an external power source. In other words, active attenuation or damping involves an externally powered attenuation or damping action taken in response to a detection indicating a swinging ofthe load, while passive attenuation or damping inherently opposes such swinging. Optionally, the damper assembly, when engaged, is configured to bias the tension in the tugger line to below a predened tension threshold. In this way, the passive attenuation can be particularly effectively congured to promote load stabilization. In this respect, it is noted that some level oftension is generally desired during use ofa tugger line, whereas excessive tension tends to be associated with erratic motions. For example, when the load swings away from the winch, tension in the tugger line may peak when the load reaches an imposedmaximum distance from the winch. At that time, the high tension tends to pull the loadback towards the winch, thereby continuing the undesired swinging. When the tension is biased to below the predened tension threshold, i.e. away from said threshold towards lower tension, such continued swinging can be suppressed. Optionally, the damper assembly, when engaged, is configured to reduce, in particular passively, an effective reeving length for the tugger line when the tension in the tugger line exceeds a predened tension threshold, e.g. the predefined tension threshold mentioned above. By reducing the effective reeving length, i.e. the length ofthe portion ofthe line that is held in the reeving, the tension in the tugger line can effectively be limited or reduced, in particular in case ofthe load swinging away from the winch. Essentially, the reduction in effective reeving length can provide some increase ofthe working length ofthe tugger line, so that a lower tension can be obtained for a same pulling force from the load, or a same tension can be maintained for an increased pulling force from the load, etc. Optionally, the damper assembly, when engaged, is additionally congured to increase, in particular passively, the effective reeving length for the tugger line when the tension in the tugger line is below a predened tension threshold. The predened tension threshold for increasing the reeving length is preferably the same as or lower than the predefined tension threshold for reducing the reeving length. In this way, overall attenuation oftension uctuations can be enhanced, wherein for example a desiredminimum tension can be maintained. Optionally, the damper assembly is congured to attenuate the uctuations using a passively operated hydraulic or pneumatic circuit ofthe damper assembly. Optionally, the damper assembly is a hydraulic or pneumatic damper assembly. Advantageously, a hydraulic or pneumatic circuit can facilitate well-regulated passive attenuation, in particular in a relatively efcient and robust manner.A hydraulic circuitmay be particularly preferred in case of relatively large loads, whereas a pneumatic circuit may be preferred in some situations where use ofhydraulic uid is undesired. Optionally, the damper assembly comprises a piston-cylinder assembly comprising a cylinder and a piston movable in the cylinder along a cylinder axis ofthe cylinder. Such a piston-cylinder assembly can provide an effective interface between the tugger line and a hydraulic or pneumatic circuit for the passive attenuation. For example, as explained further elsewhere herein, the piston- cylinder assemblymay be coupled to a reeving for the tugger line, such that relative movement ofthe piston and the cylinder varies an effective reeving length for the tugger line. Optionally, one ofthe piston and the cylinder is engaged or engageable with the tugger line, wherein preferably the other ofthe piston and the cylinder is fixed or xable in the offshore crane system, in particular to a, e.g. the, load bearing part thereof. By the engagement with the tugger line, forces can be exchanged between the piston-cylinder assembly and the tugger line. By the fixation to the load bearing part, the load bearing part can provide an effective counter force, and more generally the piston-cylinder assembly can be stably positioned. For example, the piston-cylinder assembly may be xed to a same load bearing part as the winch, andmay thus be positioned close to the winch. Nevertheless, it shall be appreciated that the winch and the damper assembly need not necessarily be close to each other, in particular because the passive attenuation can in principle be performed anywhere along the tugger line between the winch and the load. Optionally, the damper assembly is arranged to change an effective reeving length for the tugger line by movement ofthe piston with respect to the cylinder along the cylinder axis, in particular in passive response to a change in the tension in the tugger line. As alluded to above, by thus coupling the piston-cylinder assembly to a reeving for the tugger line, an effective interface between the tugger line and the hydraulic or pneumatic circuit can be realized for the passive attenuation. Using the reeving, a tension in the tugger line may be exerted as an axial force on the piston-cylinder assembly, urging relative movement ofthe piston and the cylinder, which relative movement in turn can change the effective reeving length for the tugger line at the damper assembly. In this way, depending on the configuration ofthe hydraulic or pneumatic circuit, an increase in tension in the tugger line can be passively converted to a reduction in effective reeving length and thus an increase in working length ofthe tugger line, in turn resulting in limitation or reduction ofthe tension. Optionally, an end ofthe cylinder where the piston can move towards to reduce the effective reeving length is dened as a high-pressure end, wherein an opposite end ofthe cylinder where the piston can move towards to increase the effective reeving length is defined as a low-pressure end. In this way, as explained further below, the piston-cylinder assembly can be effectively configured as part ofthe hydraulic or pneumatic circuit to provide the desired passive attenuation functionality. Optionally, the damper assembly comprises a reliefvalve arranged to drain hydraulic or pneumatic uid from the high-pressure end ofthe cylinder as pressure ofsaid uid exceeds a predened pressure threshold, in particular while otherwise blocking said draining. In this way, the above described functionality regarding the predefined tension threshold can be realized particularly effectively using the hydraulic or pneumatic circuit. For example, the reliefvalvemay remain closed as long as the tension in the tugger line is below the predefined tension threshold, allowing the tension in the tugger line and thus the uid pressure at the high-pressure end ofthe cylinder to increase up to said threshold. Then, upon a further increase, the reliefvalve opens, e.g. proportionally to the excessive pressure, thereby suppressing the further increase ofpressure and allowing the piston to move further towards the high-pressure end ofthe cylinder so as to reduce the effective reeving length ofthe tugger line. Thus, as part ofthe hydraulic or pneumatic circuit that is coupled to the tugger line using the piston-cylinder assembly, the reliefvalve can essentially facilitate that the effective reeving length ofthe tugger line is reduced in passive response to excessive tension in the tugger line, in particular while otherwise inhibiting such a reduction in effective reeving length. Optionally, the damper assembly comprises a one-way valve arranged in parallel to the reliefvalve to allow hydraulic or pneumatic uid to ow into the cylinder at the high-pressure end, in particular while preventing draining ofhydraulic or pneumatic uid from the high-pressure end otherwise than via the reliefvalve. Such a one-way valve advantageously allows relling ofthe cylinder with hydraulic or pneumatic uid at the high-pressure end to allow the piston-cylinder assembly to return to a neutral state after the piston has been moved towards the high-pressure end. As a possible alternative to the one-way valve, the reliefvalve may be adjustable, e.g. electromechanically, to selectively allow such relling ofthe cylinder. It shall be appreciated that in such a case, despite a possible active adjustment ofthe reliefvalve, the attenuation ofthe tension uctuations is still passive. In particular, the adjustment ofthe reliefvalve does not actively drive the cylinder with respect to the piston. Optionally, the damper assembly is congured to allow hydraulic or pneumatic uid toow into and out from the low-pressure end ofthe cylinder, in particular substantially without requiring any minimum pressure ofsaid uid. In this way, pressure at the low-pressure end can be substantially unaffectedby displacement ofthe piston with respect to the cylinder. Specically, such displacements can be accompanied by corresponding changes in lling ofthe cylinder at the low-pressure end. This allows the movement ofthe piston with respect to cylinder in response to changes in tension in the tugger line to be dominated by the pressure at the high pressure end. Optionally, the damper assembly comprises an accumulator arranged to store hydraulic or pneumatic uid therein, in particular for exchange thereofwith the cylinder. Such an accumulator can enable the cylinder to remain filled and appropriately pressurized, in particular without requiring active components such as a pump. A further aspect provides a combination ofone or more tugger line systems as described herein and the offshore crane system. Optionally, for at least one ofthe one or more tugger line systems, the winch is xed in the offshore crane system, in particular to the load bearing part. Optionally, for at least one ofthe one or more tugger line systems, the damper assembly is engaged with the tugger line. Optionally, for at least one ofthe one or more tugger line systems, the tugger line is engaged with the load. Advantages ofsuch a combination correspond to those described above for the tugger line system. Optionally, for at least one ofthe one or more tugger line systems, the load bearing part is, or is part of, a gantry trolley ofa gantry crane of the offshore crane system. Optionally, for at least one ofthe one or more tugger line systems, the load bearing part is, or is part of, a crane boom of the offshore crane system. Optionally, for at least one ofthe one or more tugger line systems, the load bearing part is, or is part of, a crane pedestal ofthe offshore crane system. Optionally, for at least one ofthe one or more tugger line systems, the load bearing part is, or is part of, a part ofan offshore structure, such as a vessel or a platform, on which a movable part ofthe offshore crane system is supported. Optionally, the load is, comprises, or is part of, a monopile or other component for an offshore wind turbine or a foundation therefor. Optionally, the load is, comprises, or is part of, a hammer tool for hammering a monopile for an offshore wind turbine into a seabed. Optionally, the combination is congured for the tugger line of at least one ofthe one or more tugger line systems to be engaged with the load via at least one of: a crane block ofthe offshore crane system; a monopile upending tool, in particular a ange monopile upending tool; a line connector attached to, or formed as part of, the load. Thus, the improved tugger line system can be used in a highly versatile manner in the context of offshore crane operations. Essentially, the passive attenuation can be applied for any tugger line. A further aspect provides an offshore vessel or structure comprising a combination as described herein ofthe one or more tugger line systems and the offshore crane system. Advantages ofsuch a vessel or structure correspond to those described above with respect to the tugger line system and the offshore crane system. The offshore crane system may comprise part or all ofthe vessel or structure, wherein in particular one or more xed parts ofthe vessel or structure may support one or more movable parts ofthe offshore crane system. Thus, in some cases, the vessel or structure may essentially form and / or be the offshore crane system, and vice versa. Optionally, the offshore vessel is congured for installation ofa monopile for an offshore wind turbine at an offshore installation location, and preferably congured for transport of said monopile to said offshore installation location. It has been found that the passive attenuation for the tugger lines can be particularly benecial for crane operations on such vessels. A further aspect provides a use of a tugger line system as described herein for stabilization ofa load suspended in an offshore crane system. A further aspect provides a use of a combination as described herein ofthe one or more tugger line systems and the offshore crane system for offshore crane operations, in particular for installation of a monopile for an offshore wind turbine at an offshore installation location. A further aspect provides a method of stabilizing a load suspended in an offshore crane system, comprising: engaging a tugger line with the load; using a winch, setting a working length ofthe tugger line in accordance with a desired position ofthe load; and passively attenuating uctuations oftension in the tugger line using a damper assembly engaged with the tugger line between the winch and the load. Said uctuations are in particular induced by movement ofthe load with respect to the winch. Optionally, the tugger line, winch and damper assembly are those of a tugger line system as described herein. Optionally, the passive attenuation comprises at least one of: biasing the tension in the tugger line to below a predefined tension threshold; reducing an effective reeving length for the tugger line when the tension in the tugger line exceeds a, e.g. the, predefined tension threshold; passively operating a hydraulic or pneumatic circuit comprising a piston- cylinder assembly; using a, e.g. the, piston-cylinder assembly, reducing an effective reeving length for the tugger line in passive response to an increase ofthe tension in the tugger line; using a reliefvalve, in particular in the hydraulic or pneumatic circuit, limiting an increase ofthe tension in the tugger line to a, e.g. the, predefined tension threshold. Advantages ofsuch further aspects correspond to those described above for other aspects. A further aspect provides a damper assembly evidently congured as the damper assembly ofa tugger line system as described herein. A further aspect provides a method ofproviding a tugger line system according as described herein, comprising: providing an initial tugger line system for stabilization ofa load suspended in an offshore crane system, the initial tugger line system comprising: the tugger line to be engaged with the load, and the winch xed or fixable in the offshore crane system and congured to reel the tugger line in or out from the winch so as to vary a working length ofthe tugger line; and retrofitting a damper assembly as described herein in the initial tugger line system so as to be engaged or engageable with the tugger line between the winch and the load. Optionally, the initial tugger line system is part ofan offshore crane system ofan offshore vessel or platform, in particular for installation ofmonopiles for offshore wind turbines. Thus, a damper assembly as described herein may be added to an existing tugger line system, enabling reuse ofthe existing system while also providing the additional benets ofthe passive attenuation. It shall be appreciated that aspects and options described herein may be variously combined. So, for example, options described for a system may be correspondingly applied to a related method, and vice versa. DETAILED DESCRIPTION In the following, the invention will be explained further using examples ofembodiments and drawings. The drawings are schematic and merely show examples. In the drawings, corresponding elements are provided with corresponding reference signs. In the drawings: Fig. 1 shows a wireframe perspective view ofan offshore vessel with an offshore crane system, wherein a monopile is suspended as load in the crane system; Fig. 2 shows a wireframe perspective view ofan offshore vessel with an offshore crane system, wherein ahammer tool is suspended as load in the crane system; Fig. 8 shows a side view of a tugger line system comprising a damper assembly; Fig. 4 shows a diagram ofa damper assembly in a neutral state; Fig. 5 shows a diagram ofthe damper assembly in an attenuating or damping state; Fig. 6 shows a diagram ofthe damper assembly in a recovering state; Fig. 7 shows a side view of a combination oftwo tugger line systems each comprising a respective damper assembly; and Fig. 8 shows a top view ofa further combination oftwo tugger line systems each comprising a respective damper assembly. Figs. 1 and 2 show examples ofan offshore vessel 1 in different stages of use. The shown vessel 1 is congured for installation ofa monopile 2 for an offshore wind turbine at an offshore installation location, and congured for transport ofsaid monopile 2 to said offshore installation location. In Fig. 1, a monopile 2 is suspended as load L, using both a main crane 17 and a gantry crane 12. In Fig. 2, ahammer tool 16 is suspended as load L, using the main crane 17. The shown offshore vessel 1 comprises an offshore crane system 4 and a combination ofseveral tugger line systems 8 for stabilization of a load L suspended in the offshore crane system 4. It shall be appreciated that various types and configurations of offshore crane and offshore vessel system are possible. Alternatively or additionally to an offshore vessel 1, such a combinationmay be comprisedby an offshore structure such as an offshore platform, for example. Such a combinationmay be used for offshore crane operations, in particular for installation of a monopile 2 for an offshore wind turbine at an offshore installation location. Fig. 3 shows an example ofsuch a tugger line system 3. The tugger line system 3 comprises: a tugger line 5 to be engaged with the load L; a winch 6 fixed or fixable in the offshore crane system 4, in particular to a load bearing part 7 ofthe offshore crane system 4, and congured to reel the tugger line 5 in or out from the winch 6 so as to vary a working lengthW of the tugger line 5; and a damper assembly 8 engaged or engageable with the tugger line 5 between the winch 6 and the load L. The damper assembly 8, when engaged, is configured to passively attenuate uctuations oftension in the tugger line 5. Said uctuations are inducedby movement ofthe loadL with respect to the winch 6. The hydraulic or pneumatic circuit 19 shown in different states in Figs. 4-6 is part ofthe damper assembly 8 shown in Fig. 3, even though Fig. 3 shows the piston-cylinder assembly 20 without the other parts ofthe circuit 19. Preferably, in particular when the combination ofthe crane system 4 and the tugger line system 3 is in use, the winch 6 is fixed to the load bearing part 7, the damper assembly 8 is engaged with the tugger line 5, and the tugger line 5 is engaged with the load L. Figs. 7 and 8 each show an example ofhow multiple such tugger line systems 3 can be used for a same load L, as can also be seen in Figs. 1 and 2. In the example of Fig. 7, the loadL is suspendedfrom a hoist line 9 via a crane block 10. Here, one ofthe tugger line systems 3 is engaged with the loadL via the crane block 10 while the other tugger line system 3 is engaged with the loadL more directly. In the example of Fig. 7, the respective tugger lines 5 approach the loadL from substantially opposite directions. In the example ofFig. 8, the respective tugger lines 5 approach the loadL from substantially similar directions. It shall be appreciated that various configurations ofcrane systems, tugger line systems and tugger lines are possible, including those where more than two tugger lines are used for a same load, those where one or more tugger lines extend non- horizontally, and those where tugger lines cross each other e.g. when viewed from above. With particular reference to Figs. 1 and 2 as illustrative examples, the load bearing part 7 may be, ormay be part of: a gantry trolley 11 ofa gantry crane 12 ofthe offshore crane system 4; a craneboom 13 ofthe offshore crane system 4; a crane pedestal 14 ofthe offshore crane system 4; or a part ofan offshore structure, such as a vessel 1 or a platform, on which a movable part ofthe offshore crane system 4 is supported. As examples, the loadLmay be, may comprise, ormay be part of: a monopile 2 or other component for an offshore wind turbine or a foundation therefor; or a hammer tool 16 for hammering a monopile 2 for an offshore wind turbine into a seabed. It shall be appreciated that various other types ofload bearing parts and loads are also possible. As alluded to above, a tugger line 5 may be engaged with the loadL directly or indirectly. For example, the tugger line 5 may be engaged with the loadL via: a crane block 10 ofthe offshore crane system 4; a monopile upending tool, in particular a ange monopile upending tool 18; or a line connector 28 attached to, or formed as part of, the load L. The tugger line system 5 may be realized in various ways. In some scenarios, a method ofproviding the tugger line system 5 may comprise: providing an initial tugger line system for stabilization ofa load suspended in an offshore crane system 4, the initial tugger line system comprising: the tugger line 5 to be engaged with the load L, and the winch 6 fixed or fixable in the offshore crane system 4 and congured to reel the tugger line 5 in or out from the winch 6 so as to vary a working lengthW ofthe tugger line 5; and retrotting the damper assembly 8 in the initial tugger line system so as to be engaged or engageable with the tugger line 5 between the winch 6 and the load L. The damper assembly 8 may thus initially be provided separately, for example for retrofitting or replacement purposes, or more generally as a component for eventual use as part ofthe tugger line system 3. Similar to what is described herein regarding the tugger line system 3 comprising the damper assembly 8, the initial tugger line system may be part ofan offshore crane system ofan offshore vessel or platform, in particular for installation ofmonopiles 2 for offshore wind turbines. As explained further elsewhere herein, the tugger line system 3 may be used for stabilization ofthe loadL suspended in the offshore crane system 4. Such stabilizationmay be desired in case ofa disturbance forceD acting on the load, in particular when the direction ofthe disturbance force D includes an angle with a hoist line 9 by which the loadLmay be suspended. Such a disturbance forceD can have various magnitudes and directions, can act on the loadL directly or indirectly, and can result from various sources, e.g. from wind acting on the load L, rolling or pitching of the vessel 1, or mechanical actions in or on the load L. As explained in the summary section, in the absence ofproper attenuation or damping, such disturbances tend to result in undesired swinging ofthe load. The figures also illustrate a method of stabilizing a loadL suspended in an offshore crane system 4, comprising: engaging a tugger line 5 with the load L; using a winch 6, setting a working lengthW ofthe tugger line 5 in accordance with a desired position ofthe load L; and passively attenuating uctuations oftension in the tugger line 5 using a damper assembly 8 engaged with the tugger line 5 between the winch 6 and the load L. Said uctuations are induced by movement ofthe loadL with respect to the winch 6. The tugger line 5, winch 6 and damper assembly 8 are preferably those ofa tugger line system 3 as described herein. In the shown examples, the damper assembly 8, when engaged, is congured to bias the tension in the tugger line 5 to below a predened tension threshold. The method of stabilizingmay comprise biasing the tension in the tugger line 5 to below a predened tension threshold. In the shown examples, the damper assembly 8, when engaged, is congured to reduce an effective reeving lengthR for the tugger line 5 when the tension in the tugger line 5 exceeds a, e.g. the, predened tension threshold. The method of stabilizingmay comprise reducing an effective reeving lengthR for the tugger line 5 when the tension in the tugger line 5 exceeds a, e.g. the, predened tension threshold. Thereto, the damper assembly 8 may provide and / or be coupled to a reeving for the tugger line 5, wherein the effective reeving lengthR can be understood as a variable length ofthe tugger line 5 present in the reeving. In Fig. 3 it can be seen as example that three sheaves 29 provide such a reeving, wherein distances between the sheaves 29 depend on relative positioning ofthe piston 22 and cylinder 2 1, as explained further elsewhere herein. In the shown examples, the damper assembly 8 is configured to perform the passive attenuation using a passively operated hydraulic or pneumatic circuit 19 ofthe damper assembly. In the shown examples, the damper assembly 8 is a hydraulic or pneumatic damper assembly. In the shown examples, the damper assembly 8 comprises a piston- cylinder assembly 20 comprising a cylinder 2 1 and a piston 22 movable in the cylinder 2 1 along a cylinder axis C ofthe cylinder 2 1. In the shown examples, one ofthe piston 22 and the cylinder 2 1 is engaged or engageable with the tugger line 5, wherein preferably the other ofthe piston 22 and the cylinder 2 1 is xed or fixable in the offshore crane system 4, in particular to a, e.g. the, load bearing part 7 thereof. In the shown examples, the damper assembly 8 is arranged to change an effective reeving lengthR for the tugger line 5 by movement ofthe piston 22 with respect to the cylinder 2 1 along the cylinder axis C, in particular in passive response to a change in the tension in the tugger line. The method of stabilizingmay comprise passively operating a hydraulic or pneumatic circuit 19 comprising a piston- cylinder assembly 20. The method of stabilizingmay comprise, using a, e.g. the, piston-cylinder assembly 20, reducing an effective reeving lengthR for the tugger line 5 in passive response to an increase ofthe tension in the tugger line 5. In the shown examples, an end 23 ofthe cylinder 2 1 where the piston 22 can move towards to reduce the effective reeving lengthR is dened as a high-pressure end 23, wherein an opposite end 24 ofthe cylinder 2 1 where the piston 22 can move towards to increase the effective reeving lengthR is defined as a low-pressure end 24. In the shown examples, the damper assembly 8 comprises a reliefvalve 25 arranged to drain hydraulic or pneumatic uid from the high-pressure end 23 ofthe cylinder 2 1 as pressure of saiduid exceeds a predefined pressure threshold, in particular while otherwise blocking said draining. The method of stabilizingmay comprise, using a reliefvalve 25, in particular in the hydraulic or pneumatic circuit 19, limiting an increase ofthe tension in the tugger line 5 to a, e.g. the, predefined tension threshold.A suitable value for such a predened pressure thresholdmay be determined using a model- based calculation and / or routine experimentation, in particular in dependence ofrelevant factors such as a desired level of attenuation and / or an allowable line pull on the load. In the shown examples, the damper assembly 8 comprises a one- Way valve 26 arranged in parallel to the reliefvalve 25 to allow hydraulic or pneumatic uid to ow into the cylinder 2 1 at the high-pressure end 23, in particular while preventing draining ofhydraulic or pneumatic uid from the high-pressure end 23 otherwise than via the reliefvalve 25. As a possible alternative to the one-way valve 26, the reliefvalve 25 may be adjustable, e.g. electromechanically, to selectively allow hydraulic or pneumatic uid to ow into the cylinder 2 1 at the high-pressure end 23. In the shown examples, the damper assembly 8 is configured to allow hydraulic or pneumatic uid to ow into and out from the low- pressure end 24 ofthe cylinder 2 1, in particular substantially without requiring any minimum pressure of said uid. In the shown examples, the damper assembly 8 comprises an accumulator 27 arranged to store hydraulic or pneumatic uid therein, in particular for exchange thereofwith the cylinder 2 1.A uidic connection between the accumulator 27 and the low-pressure end 24 ofthe cylinder is preferably valveless and preferably has a low uidic resistance so as to allowow between the accumulator 27 and the low-pressure end 24 substantially without requiring any minimum pressure as mentioned above. Operation ofthe damper assembly 8 including the hydraulic or pneumatic circuit 19 may be particularly well understood with reference to Figs. 4-6 illustrating the circuit 19 in different states, as explained further below. Meanwhile, Fig. 3 coupling between the piston-cylinder assembly 20 and the tugger line 5 can be understood from Fig. 3, wherein in particular the effective reeving lengthR can be understood to be dependent on the relative position ofthe piston 22 and the cylinder 2 1, and the tension T in the tugger line 5 can be understood to act on the piston 22. In Fig. 4, the circuit 19 is shown in a neutral state, wherein a non- zero but not excessive tension T in the tugger line 5 acts on the piston 22 so as to urge the piston 22 towards the high-pressure end 23 ofthe cylinder 2 1. This is counteractedby a static uid pressure in the cylinder 2 1 at the high- pressure end 23. At the low-pressure end 24, the cylinder 2 1 is at a lower static uid pressure. Here, the piston 22, and indeed the damper assembly 8 overall, is essentially at an equilibrium. It is considered that this is essentially due to a combination ofthe tension T acting on the piston 22, the action ofthe reliefvalve 25, and a difference in surface area between the sides ofpiston 22 facing the high-pressure end 23 and the low-pressure end 24 ofthe cylinder 2 1. Additionally, gravity acting on the piston 22 may contribute to the equilibrium. In Fig. 5, the tension T has increased, for example due to a disturbance D. As a result, the damper assembly 8 is brought out ofthe equilibrium. Specically, the tension T leads to increaseduid pressure at the high-pressure end 23. Upon reaching a predetermined pressure threshold, the reliefvalve 25 responds by allowing uid to drain from the high-pressure end 23 towards the accumulator 27, which in turn allows the piston 22 to move towards the high-pressure end 23, thereby reducing the effective reeving length R. The reduction in the effective reeving lengthR tends to suppress the increase in tension T, thus attenuating, in particular damping out, peaks in the tension T. As the piston 22 moves towards the high-pressure end 23, additional uid can ow into the cylinder 21 at the low-pressure end 24 from the accumulator 27. In Fig. 6, the tension T has decreased again, for example to about the same level as in Fig. 4. This can be the result ofthe attenuation, or of the disturbanceD subsiding, or both. At this time, the piston 22 may be returned towards its initial position under uid pressure from the high- pressure end 23. Specifically, once such pressure decreases below the threshold, the reliefvalve 25 will close, leaving the uid between the piston 22 and the high-pressure end 23 essentially trapped there at a pressure that is not excessive but still higher than in the neutral state of Fig. 4. Eventually, using the non-return valve 26 and the pressure present in the accumulator 27 , the neutral state ofFig. 4 can be reached again. Thus, the effective reeving lengthR is then increased again compared to the state of Fig. 5 so as to promote that aminimum tension is maintained in the tugger line 5, in particular in combination with operation ofthe winch 6. In view ofthe above, it shall be appreciated that such operation of the damper assembly 8, in particular the hydraulic or pneumatic circuit 19, can be performed passively, wherein essentially all energy for the attenuation is extracted from the peaks in tension in the tugger line 5. Over time, the hydraulic or pneumatic uid may be heatedby friction as it ows through the circuit 19, in particular at the reliefvalve 25. Such heat may be dissipated to the environment ofthe damper assembly 8 passively. Alternatively or additionally, an active cooling arrangement may be provided. It shall be appreciated that, even ifthe hydraulic or pneumatic uid is actively cooled, the attenuation by the damper assembly 8 can still be passive attenuation, in particular because such a cooling function is merely auxiliary to the attenuation and not part ofthe actual attenuation itself. Advantageously, while the damper assembly 8 provides the passive attenuation, the winch 6 may be used, in particular controlled, to inhibit the piston 22 from reaching the end of its stroke with respect to the cylinder 2 1. In particular, the winch 6 may be adjusted to increase or decrease the working lengthW ofthe tugger line 5 so as to promote that the tension T is maintained within a range at which the piston 22 is neither in contact with the high-pressure end 23 nor in contact with the low-pressure end 24 ofthe cylinder 2 1. Although the invention has been explained herein using examples ofembodiments and drawings, these do not limit the scope ofthe invention as definedby the claims. Within said scope, many variations, combinations and extension are possible, as shall be appreciated by the skilled person having the benefit ofthe present disclosure. For example, the winch as described hereinmay be operated to provide additional attenuation of tension uctuations, in particular with respect to relatively low-frequency oscillations. More generally, active attenuation oftension uctuations, including optionally using the damper assembly, may be applied in addition to the passive attenuation as described herein. Attenuation oftension uctuations may comprise pre-tensioning and / or pre-relaxing ofthe tugger line in anticipation ofupcoming uctuations inducedby movement ofthe load with respect to the winch, e.g. shortly before operation ofthe winch. All such variants are included within the scope ofthe invention as dened by the claims. LISTOF REFERENCE SIGNS 1. Offshore vessel 2. Monopile 3. Tugger line system 4. Offshore crane system 5. Tugger line 6. Winch 7. Load bearing part 8. Damper assembly 9. Hoist line 10. Crane block 1 1. Gantry trolley 12. Gantry crane 13. Craneboom 14. Crane pedestal 15. Deck 16. Hammer tool 17. Main crane 18. Flange monopile upending tool 19. Hydraulic or pneumatic circuit 20. Piston-cylinder assembly 2 1. Cylinder 22. Piston 23. High-pressure end 24. Low-pressure end 25. Reliefvalve 26. One-way valve 27. Accumulator 28. Line connector 29. Sheave 30. Line connecting load to crane block C. Cylinder axis D. Disturbance force on load L. Load R. Effective reeving length for tugger line T. Tension in tugger line W. Working length oftugger line

Claims

1. Towing line system for stabilization of an offshore crane system suspended load, comprising: - a tow line to be engaged with the load; - an anchorable winch fixed in the offshore crane system that is designed to winch the tow line from the winch in or out in order to a to vary the working length of the drawbar; and - a damper assembly engaging between the winch and the load, or engageable, is with the line of pull, whereby the damper assembly, when engaged, is designed to be passive to mitigate fluctuations of stress in the tension line, whereby the said Fluctuations are induced by movement of the load relative to the lyre.

2. Conveyor system according to claim 1, where the damper assembly, when engaged, is configured to transfer the stress in the tension line to to allow to tend below a predetermined stress threshold.

3. Conveyor line system according to claim 1 or 2, where the damper assembly, when engaged, is designed to have an effective to reduce shear length for the pull line when the tension in the tension line is greater than a predetermined stress threshold, e.g. the predetermined certain stress threshold of claim 2.

4. Conveyor line system according to one of the preceding claims, whereby The damper assembly is designed to attenuate the fluctuations with use of a passively operating hydraulic or pneumatic circuit of the damper assembly, and / or wherein the damper assembly is a hydraulic or pneumatic damper assembly.

5. Conveyor system according to claim 4, where the damper assembly a piston-cylinder assembly comprises a cylinder and a inside the cylinder encloses a movable piston along a cylinder centerline of the cylinder.

6. Conveyor line system according to claim 5, where one of the piston and the cylinder is engaged, or can be brought into engagement, with the line of pull, whereby preferably the other of the piston and the cylinder fixed or is fixable in the offshore crane system.

7. Conveyor system according to claim 6, where the damper assembly is configured to change an effective shearing length for the draw line by movement of the piston relative to the cylinder along the cylinder centerline, in particular in passive response to a change in the tension in the tension line.

8. Conveyor line system according to claim 7, where an end of the cylinder towards which the piston can move to adjust the effective shaving length to reduce is defined as a high-pressure end where a opposite end of the cylinder towards which the piston can move to to increase the effective shaving length is defined as a low-pressure end.

9. Conveyor system according to claim 8, where the damper assembly comprises a pressure relief valve designed to operate hydraulically or pneumatically to let u'1dum flow away from the high-pressure end of the cylinder as pressure of said volume a predetermined pressure threshold exceeds, in particular while the pressure relief valve is mentioned as flowing away in other cases blocks.

10. Conveyor line system according to claim 9, where: - the damper assembly comprises a one-way valve that is parallel to The pressure relief valve is designed to hydraulically or pneumatically discharge the to allow flow into the cylinder at the high-pressure end, in particular while the one-way valve flowing away from hydraulic or pneumatic fluid prevents from the high-pressure end other than via the pressure relief valve; and / or - the pressure relief valve is adaptable for hydraulic or pneumatic operation to allow the fluid to flow selectively into the cylinder at the high-pressure end. 1 1. Conveyor line system according to one of the conclusions 8 10, where the The damper assembly is designed to hydraulically or pneumatically to allow the low-pressure end of the cylinder to flow in and out, in particular essentially without requiring any minimum pressure of the said substance.

12. Conveyor line system according to one of the conclusions 4 1 1, where the damper assembly comprises an accumulator that is designed to hydraulically or to store pneumatic fluid therein, in particular for exchange of which with the cylinder as defined in claim 5.

13. Combination of one or more drawbar systems according to one of the previous conclusions and the offshore crane system.

14. Combination according to claim 13, where at least one of the the following options apply to at least one of the one or more towline systems: - the winch is fixed to the load-bearing part; - the damper assembly is engaged with the tension line; - the tow line is in action with the load.

15. Combination within the meaning of claim 13 or 14, where, for at least one of the one or more tension line systems, is the load-bearing part, or part constitutes: - a portal carriage of an offshore gantry crane crane system; - a crane boom of the offshore crane system; - a crane pedestal of the offshore crane system; - a part of an offshore structure, such as a vessel or platform, to which a movable part of the offshore crane system is attached is supported.

16. Combination according to one of the conclusions 13 15, where the burden is, comprises, or forms part of: - a monopile or other component for an offshore wind turbine or a foundation for it; or - a hammering tool for hammering a into a seabed Monopile for an offshore wind turbine.

17. Combination in accordance with one of the conclusions 13 16, arranged to the tow line of at least one of the one or more tow line systems in allow there to be engagement with the load via at least one of: - a crane block of the offshore crane system; - a monopile erection device, in particular an ens- monopile erection device; - a line connector attached to, or formed as part of, the load.

18. Offshore vessel or structure comprising a combination according to one of the conclusions 13 17.

19. Offshore vessel within the meaning of claim 18, equipped for installation of a monopile for an offshore wind turbine at an offshore installation location, and preferably equipped for the transport of the said monopile to named offshore installation location.

20. Use of a drawbar system in accordance with one of the conclusions 1 12 for stabilization of a load suspended in an offshore crane system. 2 1. Use of a combination pursuant to one of the claims 13 17 for offshore crane operations, in particular for the installation of a monopile for an offshore wind turbine at an offshore installation location.

22. Method for stabilizing a in an offshore crane system suspended load, comprising: - allowing a tow line to engage with the load; - setting a working length of the using a winch tension line corresponding to a desired position of the load; and - the passive attenuation of stress fluctuations in the tension line using a damper assembly that engages with the traction line between the winch and the load, whereby said fluctuations are induced by movement of the load relative to the winch.

23. Method according to conclusion 22, whereby the towline, the winch and the damper assembly that are part of a traction system according to one of the conclusions 1 12.

24. Method according to claim 22 or 23, whereby the passive attenuation comprises at least one of: - tilting the tension in the tension line downwards predetermined voltage threshold; - reducing the effective shearing length for the draw line when the stress in the tension line is greater than a, e.g. the, previously certain voltage threshold; - increasing the effective shearing length for the draw line when the stress in the tension line is less than a, e.g. the, previously certain voltage threshold; - the passive operation of a hydraulic or pneumatic circuit comprising a piston-cylinder assembly; - the using of a, e.g. the, piston-cylinder assembly reducing the effective shear length for the draw line in passive reaction to an increase in stress in the tension line; - by using a pressure relief valve, in particular in the hydraulic or pneumatic switching, limiting an increase of the stress in the tension line to a, e.g. the, predetermined voltage threshold.

25. Threshold assembly apparently configured as the damper assembly of a drawbar system according to one of the conclusions 1 12.

26. Procedure for providing a towline system according to one of the claims 1 12, comprising: - providing an initial tension line system for stabilization of a load suspended in an offshore crane system, where the initial drawbar system comprises: the drawbar to be engaged with the load, and the lockable winch fixed in the offshore crane system that is configured to winch the tow line from the winch in or out to achieve a working length of the to vary the drawbar; and - retrofitting a damper assembly in accordance with claim 28 in the initial drawbar system to engage, or to bring into engagement, to be with the tow line between the winch and the load.

27. Method according to conclusion 26, whereby the initial tow line system forms part of an offshore crane system of a offshore vessel or platform, in particular for the installation of monopiles for offshore wind turbines.

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