Cassette assembly, surface boat, maintenance method of water accessible structure

Abstract

To provide a cassette assembly for a surface boat, the surface boat for supporting the cassette assembly, and the maintenance method of water accessible structures such as a wind turbine.SOLUTION: A cassette assembly (200) can be installed to an end (102) of a surface boat (100) and includes a cassette part (210) abutting against an external structure during usage. The cassette part (210) is movable to the opposite end direction of the boat (100) with respect to the boat (100). An elastic cushioning system (250) counters the movement of the cassette part (210) when the boat (100) moves toward the external structure. The elastic cushioning system (250) includes a fluid operated shock absorber (251). The surface boat (100) includes a receiving part (120) for receiving the cassette assembly (200) and can be used for maintaining the water accessible structure such as a wind turbine.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a cassette assembly for a surface vessel, a surface vessel for supporting the cassette assembly, and a method for servicing a water-accessible structure such as a wind turbine. [Background technology]

[0002] Various challenges arise when servicing a water-accessible structure, such as an offshore wind turbine with a surface vessel. One difficulty is allowing personnel to access the structure. In one example, a vessel may be purposefully navigated into a water-accessible structure, bringing the vessel into contact with the structure and allowing access to the structure. However, significant impact forces may be exerted on the structure and / or the vessel, potentially damaging the structure and / or the vessel.

[0003] GB 2390252 A1 discloses a surface vessel having a carriage assembly at one end of the vessel. The carriage assembly is moveable relative to the vessel and is shock absorbed so that impact forces are absorbed when the carriage assembly impacts a water-accessible structure. In order to safely use such a system without causing damage to either the vessel or the structure, constraints are imposed on its movement. This may include limiting the mass of the vessel incorporating the carriage assembly, the impact speed, and the impact angle of the vessel relative to the structure. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] GB 2390252 Summary of the Invention [Problem to be solved by the invention]

[0005] It is an object of the present invention to address these and any other problems which will become apparent to those skilled in the art from the disclosure herein. [Means for solving the problem]

[0006] According to the present invention there is provided an apparatus and method as set out in the accompanying claims. Further features of the invention will become apparent from the dependent claims and the following description.

[0007] According to a first aspect of the present disclosure, there is provided a cassette assembly mountable to one end of a surface vessel, comprising: a cassette part which in use abuts the external structure and which is movable relative to the vessel towards an opposite end of the vessel; a resilient shock absorber system for resisting movement of the cassette portion when the vessel is moved towards the external structure; Equipped with A resilient shock absorber system is provided for the cassette assembly, which includes a fluid-actuated shock absorber.

[0008] The cassette part may be rotatable, preferably in a horizontal plane.

[0009] The cassette part may include a passageway formed in an upper surface thereof.

[0010] The resilient dampening system may comprise at least two dampeners spaced across the width of the cassette portion.

[0011] The cassette part may comprise a fender part configured to contact the outer structure. The fender part may be formed of a resilient material. The fender part may comprise a central projection and a pair of recesses located on respective lateral sides of the central projection. The recesses may comprise a stop surface to prevent outboard movement of the first support tube of the outer structure beyond the stop surface. The recesses and the central projection may be configured such that contact between the first support tube and the stop surface causes rotation of the cassette part such that the fender part, suitably the central projection, contacts the second support tube of the outer structure.

[0012] The external structure may be an offshore wind turbine. The external structure may be an offshore platform or other fixed or floating structure.

[0013] The shock absorber may be rotatably mounted to the cassette part, suitably to allow rotation in a horizontal plane.The shock absorber may be rotatably mounted to the vessel, suitably to allow rotation in a horizontal plane.

[0014] The fluid-operated shock absorber may be a hydraulic shock absorber.The fluid-operated shock absorber may be a pneumatic shock absorber.

[0015] The cassette assembly may comprise a fluid actuating system configured to operate the fluid actuated shock absorber. The fluid actuating system may comprise an accumulator configured to supply fluid to the shock absorber, suitably to counter compression of the shock absorber. The accumulator may be a pneumatically charged accumulator. The fluid actuating system may comprise a flow control unit disposed in a fluid flow path between the shock absorber and the accumulator. The flow control unit may be configured to supply fluid from the accumulator to the shock absorber at a predetermined pressure. The flow control unit may comprise a flow restrictor. The flow control unit may comprise a check valve disposed in parallel with the flow restrictor. The fluid actuating system may comprise a pressure bypass valve disposed in the fluid flow path between the shock absorber and the accumulator. The pressure bypass valve may be configured to bypass the flow control unit in response to a pressure exceeding a predetermined bypass pressure.

[0016] The cassette assembly may include a controller configured to adjust the energy absorption and reaction force provided by the shock absorber. The controller may be configured to adjust the flow rate of the flow control unit and / or the flow rate of the accumulator.

[0017] The resilient buffer system may include a stop element configured to limit movement of the cassette part toward the opposite end of the vessel. The stop element may be formed of an elastically deformable material. The cassette part may be configured to contact the stop element before the buffer reaches a maximum compression limit.

[0018] According to a second aspect of the present disclosure, there is provided a surface vessel comprising a receiver configured to receive a cassette assembly as defined herein.

[0019] The receiver may be configured to slidably support the cassette assembly. The receiver may rotatably support the cassette assembly.

[0020] The receiver may comprise a recess within the vessel, suitably at the front of the vessel. The receiver may comprise a rear wall. The rear wall may comprise at least two mounting posts for supporting the at least two shock absorbers. The receiver may comprise substantially vertical side walls.

[0021] The receiver may extend across at least 50% of the width of the forward portion of the vessel. Suitably, the receiver may extend across at least 60% of the width of the forward portion of the vessel. The receiver may extend across at least 70% of the width of the forward portion of the vessel.

[0022] The surface vessel may include a cassette assembly disposed in the receptacle.

[0023] Further suitable features of the vessel of the second aspect are defined above in relation to the cassette assembly of the first aspect and may be combined in any combination.

[0024] According to a third aspect of the present disclosure, there is provided a method of servicing a water-accessible structure using a surface vessel comprising an end-located cassette assembly, the method comprising: contacting a cassette portion of the cassette assembly with a water-accessible structure; using an elastic buffer system to counteract relative movement of the cassette part towards the opposite end of the vessel; Equipped with A method is provided in which the resilient shock absorber system includes a shock absorber comprising a fluid-actuated shock absorber.

[0025] The water-accessible structure may be a wind turbine.

[0026] Further relevant features of the method of the second aspect are defined above in relation to the cassette assembly of the first aspect and the vessel of the second aspect and may be combined in any combination.

[0027] According to a fourth aspect of the present disclosure, there is provided a use of a cassette assembly and / or a surface vessel as defined herein.

[0028] According to a fifth aspect of the present disclosure, there is provided a cassette assembly mountable to an end of a surface vessel, the cassette assembly comprising: a cassette part which in use abuts the external structure and which is movable relative to the vessel towards an opposite end of the vessel; a resilient shock absorber system for resisting movement of the cassette portion when the vessel is moved towards the external structure; Equipped with The cassette portion is a fender portion configured to contact an external structure, A central protrusion; a pair of recesses located on each lateral side of the central projection; A cassette assembly is provided that includes a fender portion comprising:

[0029] The recess may include a stop surface to prevent overboard movement of the first support tube of the outer structure beyond the stop surface, and the recess and central projection may be configured such that contact between the first support tube and the stop surface causes rotation of the fender portion, suitably the cassette portion such that the central projection contacts the second support tube of the outer structure.

[0030] Further relevant features of the assembly of the fifth aspect are defined above in relation to the cassette assembly of the first aspect and the vessel of the second aspect and may be combined in any combination.

[0031] For a better understanding of the present disclosure, and to show how examples of the same may be put into effect, reference will now be made, by way of example only, to the accompanying schematic drawings. [Brief description of the drawings]

[0032] [Figure 1] FIG. 1 illustrates an overhead plan view of one end of an exemplary surface vessel including an exemplary cassette assembly. [Diagram 2] FIG. 2 is a side cross-sectional view of one end of the exemplary surface vessel of FIG. [Diagram 3] FIG. 1 is a schematic block diagram of an exemplary fluid-actuated shock absorber system. [Figure 4] FIG. 2 is a plan view of an exemplary fender portion. [Figure 5A] FIG. 3 is an overhead plan view of the exemplary surface vessel of FIGS. 1 and 2 in contact with a water-accessible structure. [Figure 5B] FIG. 3 is an overhead plan view of the exemplary surface vessel of FIGS. 1 and 2 in contact with a water-accessible structure. [Figure 5C] FIG. 3 is an overhead plan view of the exemplary surface vessel of FIGS. 1 and 2 in contact with a water-accessible structure. [Figure 6] 1 is a flow chart of an exemplary method for providing a water-accessible structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] In the drawings, corresponding reference characters indicate corresponding components. Those skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of the various exemplary embodiments. Also, common but well-understood elements that are useful or necessary in commercially feasible embodiments are often not depicted to facilitate a less obtrusive view of these various exemplary embodiments.

[0034] In summary, examples of the disclosure provide a cassette assembly mountable on the end of a surface vessel, the cassette assembly comprising a movable cassette portion for abutting against an external structure, and a resilient damping system for resisting movement of the cassette portion, the resilient damping system comprising a fluid-actuated damper, such as a hydraulic or pneumatic damper. In some examples, the resilient damping system comprises two dampers spaced across the width of the cassette portion. The use of the fluid-actuated dampers allows the energy absorbed by the damping system and the reaction force exerted to be tailored to suit, for example, the vessel or its intended use.

[0035] 1 and 2 show a forward portion 102 of an exemplary surface vessel 100. A longitudinal centerline of the vessel 100 is indicated by reference numeral 101. The forward portion 102 extends horizontally or laterally such that it is approximately perpendicular to the centerline 101 when viewed in a plan view.

[0036] The vessel 100, and more particularly the forward portion 102, includes a cassette assembly, generally indicated by reference numeral 200. As best seen in Figure 2, the cassette assembly 200 is disposed within a receiver 120 formed within the structure of the vessel 100. For example, the receiver 120 takes the form of a recess extending laterally across the forward portion 102 that is configured to receive the cassette assembly 200.

[0037] The receiving portion 120 is coupled vertically by a substantially horizontal deck plate of the ship 100. For example, the receiving portion 120 is coupled by an overhanging hull structure 121 above the receiving portion 120 and a hull 122 below the ship 100.

[0038] Similarly, the receiver 120 is horizontally bounded at its lateral ends by a generally vertical bulkhead. In one example, the vessel 100 is a catamaran. In such an example, a generally vertical bulkhead may be located on the centerline of each demi-hull of the vessel 100. The aft portion of the recess 120 may be defined by a generally vertical transverse bulkhead 123.

[0039] The cassette assembly 200 comprises a cassette part 210 or carriage. The cassette part 210 takes the form of a platform slidably mounted within the receiver 120 for movement in a longitudinal direction indicated by arrow A. The cassette part 210 can also rotate in a horizontal plane as indicated by arrow B. For example, the cassette part 210 may comprise arcuate portions on its lateral edges supported by corresponding arcuate channels.

[0040] The top surface 211 of the cassette portion 210 allows a user to access the water-accessible structure. For example, the walkway 220 may be formed in the top surface 211, although it will be appreciated that in some instances the entire top surface 211 may be walked on.

[0041] The forward portion 212 of the cassette portion 210 is configured to contact the water-accessible structure. The forward portion 212 (and thus the receiving portion 120) may extend approximately 65-70% of the width of the forward portion 102. As shown in FIG. 1, the forward portion 212 may include a fender portion 230. The fender portion 230 is formed of a flexible or elastic material that can be compressed upon contact with the water-accessible structure, such as rubber. The shape of the fender portion 230 is described in more detail below with reference to FIGS. 4 and 5. In some examples, the watercraft 100 may also include fender portions 105 disposed on either side of the cassette assembly.

[0042] The cassette assembly 200 also includes a resilient dampening system 250. The resilient dampening system 250 is configured to resist movement of the cassette portion 210 caused by contact between the cassette portion 210 and the water-accessible structure. The resilient dampening system 250 includes a pair of dampers 251. The dampers 251 are spaced across the width of the cassette portion 210.

[0043] The bumper 251 extends from the rear bulkhead 123 of the receiver 120 to a stop surface 213 of the cassette part 210. The stop surface 213 may be a generally vertical surface that forms one end of a horizontally extending guide channel 214 within the cassette part 210. The bumper 251 may be attached to the rear bulkhead 123 by a mounting post 252 extending horizontally from the rear bulkhead to which the bumper 251 can be secured.

[0044] Each bumper 251 comprises a fluid-actuated ram. For example, the ram 251 may be a hydraulic ram. The ram 251 is rotatably mounted to the stop surface 213 such that the ram 251 can rotate in a horizontal plane relative to the stop surface 213. Similarly, the ram 251 is rotatably mounted to the mounting post 252 such that the ram 251 can rotate in a horizontal plane relative to the mounting post 252. For example, the ram 251 may be coupled to the mounting post 252 and / or the stop surface 213 using a vertical pin, a ball and socket joint, or any other suitable means that provides the desired rotation.

[0045] The elastic buffer system 250 further comprises a stop element 258 configured to limit the movement of the cassette part 210 in the longitudinal direction A, from the forward part 102 towards the opposite end of the vessel. The stop element 258 is arranged in the receiver 120 on the rear bulkhead 123. For example, there may be two stop elements 258 for each buffer 251 arranged respectively above and below the mounting post 252. The stop element 258 is thus configured to contact the cassette part 210 in the areas above and below the channel 215. The stop element 258 is formed of a flexible or elastic material that can deform on contact. For example, the stop element 258 may be a rubber fender block.

[0046] The stop element 258 ensures that if the cassette part 210 moves into contact with the aft bulkhead 252, the impact is buffered to prevent damage to the cassette part 210 and / or the vessel 100. In one example, the cassette assembly 200 is configured such that the cassette part 210 contacts the stop element 258 before the buffer 251 reaches its maximum compression limit, thereby preventing damage to the buffer 251.

[0047] An exemplary fluid actuation system 400 for operating the ram 251 is shown in Figure 3. In the example of Figure 3, the fluid actuation system 400 is a hydraulic system. The system 400 includes the ram 251 and a hydraulic accumulator 410 configured to provide fluid to the ram 251 to counteract compression of the ram 251. The accumulator 410 may be pneumatically charged, for example by a multi-stage pump. In another example, the accumulator 410 may be a spring accumulator.

[0048] The ram 251 may be a double acting ram and the rod port of the ram 251 is connected to a fluid reservoir 420 at very low pressure to prevent rusting during use.

[0049] In one example, the system 400 also includes a flow control unit 430. The flow control unit 430 may include a flow restrictor 431, such as an orifice plate, and a check valve 432. In one example, the check valve 432 and the flow restrictor 431 are arranged in parallel. The flow control unit 430 ensures that a predetermined pressure is provided to the ram 251 and may assist in dissipating heat generated by the system 400.

[0050] The system 400 may also include a pressure bypass valve 440 fluidly connected between the ram 251 and the accumulator 410. The pressure bypass valve 440 may bypass the flow control unit 440 in the event of excess pressure caused, for example, by a hard impact on the ram 251, thereby avoiding overloading of the system 400.

[0051] The system 400 is configured to provide a resilient bias to the ram 251. Specifically, compression of the ram 251 causes hydraulic fluid within the system 400 to flow to an accumulator 410 where it is stored under pressure. The accumulator 410 then discharges pressurized fluid via a flow control unit 430 to counter the compression of the ram 251.

[0052] Additionally, the system 400 can be tuned to absorb a desired amount of energy and exert a desired counter force by adjusting parameters of the accumulator 410 and / or the flow control unit 430. For example, the flow rate or pressure provided by the accumulator 410 and / or the flow control unit 430 may be adjusted.

[0053] In one example, the parameters may be passively adjusted, in other words, the parameters may be adjusted upon installation of the system 400 on the vessel 100. This allows the system 400 to be adjusted according to the characteristics (e.g., mass) and intended use of the vessel 100 without substantially redesigning the system 400.

[0054] In another example, the parameters may be passively adjusted. For example, the system 400 may include a controller (not shown), which may include a processor and memory configured to adjust the parameters in response to user input. Thus, a user may vary the operating characteristics of the fluid actuating system 400 after installation, for example, depending on the use or loading of the vessel.

[0055] In one example, a fluid actuation system 400 is provided for each shock absorber 251. Thus, the example shown in Figure 1 may include two fluid actuation systems 400. In another example, one fluid actuation system 400 may be associated with multiple shock absorbers 251.

[0056] The example of Figure 3 is a hydraulic system in which the fluid is a liquid such as oil, however in other examples the system may be a pneumatic system in which the fluid may be a gas such as air.

[0057] 4, the shape of fender section 230 will be described in more detail. Fender section 230 includes a central projection 231 that projects longitudinally from substantially the lateral center of fender section 230. The fender section also includes two lateral projections 232a and 232b located respectively on the port and starboard sides of central projection 231. Port and starboard recesses 233a,b are located between central projection 231 and lateral projections 232a,b.

[0058] Each recess 233 has a first inclined portion 234, a second inclined portion 235, and a third inclined portion 236 arranged to extend continuously away from the central projection 231 to its respective lateral projection 232. The first inclined portion 234 inclines longitudinally inward (i.e., away from the front portion 102) as it extends laterally away from the central projection 231, and the second inclined portion 235 also inclines longitudinally inward as it extends away from the central projection 231. The inclination angle of the second inclined portion 235 is shallower than that of the first inclined portion 234. The recess 233 further comprises a third inclined portion 236 that inclines longitudinally outward (i.e., toward the front portion 102) as it extends toward the lateral projection. The angle α between the first and second inclined portions 234, 235 is about 130 degrees. The angle β between the second and third ramps 235, 236 is approximately 130 degrees.

[0059] Use of the vessel 100 and cassette assembly 200 will now be described with reference to Figures 5A-5C, which additionally show a water-accessible structure 300.

[0060] For example, the water-accessible structure may be an offshore wind turbine 300. The offshore wind turbine 300 includes an access structure 310 formed on one side thereof that allows access to the turbine 300. The access structure 310 includes a pair of spaced apart support tubes 311 and a ladder 312 disposed between the support tubes 311.

[0061] In use, the vessel approaches the turbine 300 and is navigated such that the fender portion 230 of the cassette assembly 200 impacts the support tube 311. When the fender portion 230 contacts the support tube 311, a reaction force is applied to the cassette portion 210. The reaction force biases the cassette portion 210 rearward. The resilient shock absorber system 250 absorbs the impact of the collision by compression of the shock absorber 251, thereby preventing damage to the vessel 100 and / or turbine 300.

[0062] When the vessel 100 is in contact with the turbine 300, the vessel 100 can be kept in position by applying forward thrust (e.g., via a propeller or other powered device of the vessel 100). Personnel wishing to access the turbine 300 can then board the gangway 220 and ladder 312. After the maintenance work is completed and the personnel return to the vessel 100, the vessel 100 applies reverse thrust, thereby moving away from the wind turbine 300. In doing so, the forces compressing the bumpers 251 are removed, allowing their elastic forces to re-expand the bumpers to the condition shown in FIG. 2.

[0063] In Fig. 5A, the vessel 100 is shown in collision aligned with the turbine 300 such that the longitudinal axis 101 of the vessel 100 is perpendicular to a notional line extending between the tubes 311. In this condition, the central protrusion 231 is received between the tubes 311, which contact the recesses 233a,b on either side of the central protrusion 231. This is an ideal situation in which the collision force is substantially aligned with the compression direction of the buffer 251. Furthermore, the position of the central protrusion 231 between the tubes 311 may help to hold the vessel 100 in place, for example, when wind or currents act on the vessel 100.

[0064] Figure 5B shows initial contact between the vessel 100 and the turbine 300, with the vessel 100 displaced approximately 10 degrees relative to the position shown in Figure 5A. In this displaced approach, a first one of the support tubes 311 is engaged by the port recess 233a and captured at the junction of the second and third ramps 235a, 236a. The second and third ramps 235, 236 thus act as stop surfaces, preventing further outboard movement of the first tube 311 relative to the fender section 230.

[0065] As can be seen in Figure 5C, the impact causes the port side of the cassette part 210 to move inward and rotate the cassette part 210 in a horizontal plane. This causes the central projection 231 to contact the second tube of the support tube 311. The shape of the fender part 230 therefore ensures that both tubes are in contact even in the case of an out-of-position impact. This ensures that the impact is shared between both tubes 311, preventing damage to the tubes 311. This also helps to ensure that both elastic shock absorbers 251 absorb the impact.

[0066] It will be appreciated that in some instances, the cassette assembly 200 may be retrofitted to the vessel 100 having a suitable receptacle. For example, the cassette assembly 200 may be bolted or welded to the vessel's foredeck, for example, utilizing an open space typically located on the vessel's forward cargo deck. Similarly, the cassette assembly 200 may be removable from the vessel 100, for example, for maintenance, repair, etc.

[0067] In one example, the cassette assembly may include a fluid-actuated shock absorber 251 and a shock absorber comprising a compressible element. The compressible element may comprise a fender cell or cone formed from a resiliently compressible material, such as rubber. The compressible element may be capable of being compressed to absorb the energy of contact between the cassette portion 210 and the water-accessible structure. When the compressive force is removed, the compressible elements are configured to experience a restoring force, thereby returning to their initial, substantially uncompressed state. The shock absorber comprising a compressible element may be a shock absorber comprising multiple compressible elements that may be arranged in series to form a shock absorber. The present disclosure extends to any combination of any number of fluid-actuated shock absorbers and shock absorbers formed from compressible elements, including shock absorbers comprising single or multiple compressible elements.

[0068] 6 illustrates a method of servicing a water-accessible structure using a surface vessel with a cassette assembly disposed at an end of the surface vessel. In block S601, a cassette portion of the cassette assembly is contacted with the water-accessible structure. In block S602, relative movement of the cassette portion toward the opposite end of the vessel is opposed using a resilient damping system.

[0069] Advantageously, the above-described examples provide an improved vessel for accessing underwater structures. The use of fluid-actuated shock absorbers results in a system that can be easily tuned to provide the desired energy absorption and reaction forces. Thus, substantially the same hardware can be adapted to a wide range of vessels having different masses and different operating limitations. This can avoid the need to procure different elastic compressible elements to be equipped with shock absorbers for different vessels. Furthermore, in some examples, the system can be actively tuned to provide the desired energy absorption and reaction forces to account for the characteristics of the vessel and / or the external structure being accessed. Furthermore, the use of fluid-actuated shock absorbers provides a system that can absorb more energy without a significant increase in the impact reaction forces. Thus, vessels having a larger mass can be used to service water-accessible structures and a larger tolerance in impact velocity can be tolerated. Furthermore, the above-described examples provide a vessel that can approach structures from a wider range of angles without causing damage to either the vessel or the structure, thereby facilitating maneuvering of the vessel.

[0070] Attention is directed to all articles and documents related to this application that are filed contemporaneously or prior to this application and that are open to public inspection herewith, the contents of all such articles and documents being incorporated herein by reference.

[0071] All of the features disclosed in this specification (including the accompanying claims, abstract, and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations in which at least some of such features and / or steps are mutually exclusive.

[0072] Each feature disclosed in this specification (including the accompanying claims, abstract, and drawings), unless expressly stated otherwise, may be replaced by alternative features serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

[0073] The invention is not limited to the details of the embodiment(s) described above.The invention extends to any novel, or any novel combination of features disclosed in this specification (including the accompanying claims, abstract and drawings), or to any novel, or any novel combination of steps of any method or process so disclosed.

Claims

1. 1. A cassette assembly mountable to one end of a surface vessel, comprising: a cassette part which in use abuts the external structure and which is movable relative to the vessel towards an opposite end of the vessel; a resilient shock absorber system for resisting movement of the cassette portion when the vessel is moved towards the external structure; Equipped with The cassette assembly, wherein the resilient dampening system comprises a fluid actuated dampener.

2. The cassette assembly of claim 1 , wherein the cassette section is rotatable in a horizontal plane.

3. 3. A cassette assembly as claimed in claim 1 or 2, wherein the cassette portion includes a passageway formed in an upper surface thereof.

4. A cassette assembly according to any preceding claim, comprising at least two buffers spaced across the width of the cassette part.

5. A cassette assembly according to any preceding claim, wherein the buffer is rotatably mounted to the cassette part to allow rotation in a horizontal plane.

6. A cassette assembly according to any preceding claim, wherein the fluid-actuated shock absorber is a hydraulic shock absorber.

7. 7. The cassette assembly of claim 6, further comprising a fluid actuating system configured to operate the fluid actuated shock absorber, the fluid actuating system comprising an accumulator configured to supply fluid to the shock absorber to counter compression of the shock absorber.

8. The fluid actuating system includes a flow control unit disposed in a fluid flow path between the shock absorber and the accumulator; The cassette assembly of claim 7 , wherein the flow control unit is configured to supply fluid from the accumulator to the buffer at a predetermined pressure.

9. 9. The cassette assembly of claim 8, wherein the fluid actuation system includes a pressure bypass valve disposed in the fluid flow path between the shock absorber and the accumulator, the pressure bypass valve configured to bypass the flow control unit in response to pressure exceeding a predetermined bypass pressure.

10. 6. A cassette assembly according to claim 1, wherein the fluid-actuated shock absorber is a pneumatic shock absorber.

11. A cassette assembly according to any preceding claim, comprising a controller configured to adjust the energy absorption and / or reaction force provided by the fluid-actuated shock absorber.

12. The cassette portion includes a fender portion configured to contact the external structure, the fender portion comprising: A central protrusion; a pair of recesses positioned on each lateral side of the central projection; Equipped with the recess includes a stop surface to prevent overboard movement of the first support tube of the outer structure beyond the stop surface; 12. A cassette assembly according to any one of claims 1 to 11, wherein the recess and the central protrusion are configured such that contact between the first support tube and the stop surface causes rotation of the cassette part such that the central protrusion contacts a second support tube of the external structure.

13. A cassette assembly according to any one of claims 1 to 10, wherein the resilient buffer system comprises stop elements arranged to limit movement of the cassette parts towards the opposite end of the vessel.

14. 14. The cassette assembly of claim 13, wherein the cassette parts are configured to contact the stop element before the buffer reaches a maximum compression limit.

15. 15. A surface vessel comprising a receiver configured to receive a cassette assembly as defined in any one of claims 1 to 14.

16. The surface vessel of claim 15 , wherein the receiver comprises a recess within the vessel.

17. 17. The surface vessel of claim 15 or 16, comprising a cassette assembly disposed in the receptacle.

18. 1. A method of servicing a water-accessible structure using a surface vessel, the surface vessel comprising a cassette assembly disposed at one end of the surface vessel, the method comprising: contacting a cassette portion of the cassette assembly with a water-accessible structure; using an elastic buffer system to counteract relative movement of the cassette part towards the opposite end of the vessel; Equipped with The method, wherein the resilient damping system comprises a fluid-actuated damper.

19. The method of claim 18 , wherein the water-accessible structure is a wind turbine.

20. Use of a cassette assembly according to any one of claims 1 to 14 and / or a surface vessel according to any one of claims 15 to 17.

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