CASSETTE ASSEMBLY, WATERBORNE VESSEL AND METHOD OF SERVICING A WATER-ACCESSIBLE STRUCTURE

A cassette assembly is disclosed attachable to an end portion of a waterborne vessel and including a cassette section to bear against an external structure in use. The cassette section is movable relative to the vessel in a direction towards an opposite end of the vessel. A resilient buffer system counters movement of the cassette section when the vessel is moved towards the external structure, wherein the resilient buffer system includes a buffer including a plurality of resiliently compressible elements arranged in series. A waterborne vessel includes a receiving portion for receiving the cassette assembly, and can be used to service a water-accessible structure such as a wind turbine.

FIELD

The present disclosure relates to a cassette assembly for a waterborne vessel, a waterborne vessel to support a cassette assembly, and a method of servicing a water-accessible structure, such as a wind turbine.

BACKGROUND

Various challenges arise in servicing water-accessible structures such as offshore wind turbines with waterborne vessels. One difficulty is that of allowing personnel access to the structure. In one example, the vessel may be deliberately navigated into the water-accessible structure, bringing the vessel into contact with the structure and allowing access thereto. However, a significant impact force can be imparted to the structure and/or the vessel, potentially damaging the structure and/or vessel.

UK Patent Publication No. GB 2390252 A discloses a waterborne vessel with a carriage assembly at one end of the vessel. The carriage assembly is movable relative to the vessel and buffered such that, when the carriage assembly impacts the water-accessible structure, the impact force is absorbed. In order to safely use such a system without damaging either the vessel or the structure, constraints are placed on the operation thereof. This may include limiting the mass of the vessel incorporating the carriage assembly, the impact velocity and the angle of impact of the vessel to the structure.

It is an aim of the invention to address these difficulties, and any other difficulties that would be apparent to the skilled reader from the disclosure herein.

SUMMARY

According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

According to a first aspect of the disclosure there is provided a cassette assembly attachable to an end portion of a waterborne vessel, the cassette assembly comprising:a cassette section to bear against an external structure in use, the cassette section movable relative to the vessel in a direction towards an opposite end of the vessel; anda resilient buffer system to counter movement of the cassette section when the vessel is moved towards the external structure,wherein the resilient buffer system comprises a buffer comprising a plurality of resiliently compressible elements arranged in series.

The cassette section may be rotatable, suitably in a horizontal plane.

The cassette section may comprise a walkway formed on an upper surface thereof.

The resilient buffer system may comprise at least two buffers spaced apart across a width of the cassette section. Each buffer of the resilient buffer system may comprise a plurality of resiliently compressible elements arranged in series.

The cassette section may comprise a fender section configured to contact the external structure. The fender section may be formed of a resilient material. The fender section may comprise a central protrusion, and a pair of recesses positioned at respective transverse sides of the central protrusion. The recesses may comprise a stop surface, to prevent outboard motion of a first support tube of the external structure beyond the stop surface. The recesses and central protrusion may be configured so that contact between the first support tube and the stop surface causes rotation of the cassette section such that the fender section, suitably the central protrusion, contacts a second support tube of the external structure.

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

The resiliently compressible elements may be tubular. The compressible elements may comprise cell fenders. Each compressible element may have a longitudinal length of from 250 mm to 1000 mm, suitably from 450 mm to 800 mm, suitably approximately 500 mm. The diameter of the tubular portion of the compressible element may be from 250 mm to 1000 mm, suitably from 450 mm to 800 mm, suitably approximately 550 mm. The compressible element may have a maximum compression of over 20%, suitably over 30%, suitably over 40%, suitably over 50%, most suitably approximately 52.5%. Compression of the compressible element may require 19-50 kNm energy, suitably 20-40 kNm, suitably 22-30 kNm, suitably 23-28 kNm, suitably approximately 26 kNm. Compression of the compressible element may provide a reaction force of approximately 90-235 kN, suitably 100-180 kN, suitably 110-140 kN, suitably approximately 120 kN.

The resilient buffer system may comprise a vertical guide to restrict vertical movement of the buffer, suitably to prevent vertical movement of the buffer. The vertical guide may comprise one or more guide projections formed on one of the buffer or a guide channel of the cassette section, configured to cooperate with a slot formed on the other of the buffer or the guide channel. The projection may project radially from the buffer. The projection may be disposed between neighbouring compressible elements.

The resilient buffer system may comprise a transverse guide to limit a range of transverse movement of the buffer. The transverse guide may comprise first contact elements formed on transverse edges of the buffer, the first contact elements configured to contact second contact elements formed on transverse edges of the guide channel. The guide channel may be sized to permit a predetermined range of transverse motion of the buffer.

Each buffer may comprise three compressible elements arranged in series. Each buffer may comprise 4, 5, 6 or more compressible elements arranged in series.

The resilient buffer system may comprise stop elements configured to limit the motion of the cassette section towards the opposite end of the vessel. The stop elements may be formed of a resiliently deformable material. The cassette section may be configured to contact the stop elements before the buffers reach a maximum compression limit.

The cassette assembly may include an over rotation prevention unit, configured to prevent excessive rotation of the cassette section. The over rotation prevention unit may comprise a plurality of restraining ties, configured to prevent motion of the cassette section stretching the buffers beyond an uncompressed position.

According to a second aspect of the disclosure there is provided a waterborne vessel comprising a receiving portion, the receiving portion configured to receive a cassette assembly as defined herein.

The receiving portion may be configured to slidably support the cassette assembly. The receiving portion may rotatably support the cassette assembly.

The receiving portion may comprise a recess in the vessel, suitably in a front portion of the vessel. The receiving portion may comprise a rear wall. The rear wall may comprise at least two mounting posts to support the at least two buffers. The receiving portion may comprise substantially vertical side walls.

The receiving portion may extend across at least 50% of the width of a forward portion of the vessel. Suitably, the receiving portion may extend across at least 60% of the width of a forward portion of the vessel. The receiving portion may extend across at least 70% of the width of a forward portion of the vessel.

The waterborne vessel may comprise a cassette assembly disposed in the receiving portion.

Further suitable features of the vessel of the second aspect are defined hereinabove in relation to the cassette assembly of the first aspect, and may be combined in any combination.

According to a third aspect of the disclosure there is provided a method of servicing a water-accessible structure using a waterborne vessel, the waterborne vessel comprising a cassette assembly arranged at an end portion of the waterborne vessel, the method comprising:bringing a cassette section of the cassette assembly into contact with the water accessible structure, andcountering relative movement of the cassette section in a direction towards an opposite end of the vessel using a resilient buffer system,wherein the resilient buffer system comprises a buffer comprising a plurality of resiliently compressible elements arranged in series.

The water-accessible structure may be a wind turbine.

Further suitable features of the method of the second aspect are defined hereinabove in relation to the cassette assembly of the first aspect and vessel of the second aspect, and may be combined in any combination.

According to a fourth aspect of the disclosure there is provided use of a cassette assembly and/or waterborne vessel as defined herein.

According to a fifth aspect of the disclosure there is provided a cassette assembly attachable to an end portion of a waterborne vessel, the cassette assembly comprising:a cassette section to bear against an external structure in use, the cassette section movable relative to the vessel in a direction towards an opposite end of the vessel; anda resilient buffer system to counter movement of the cassette section when the vessel is moved towards the external structure,wherein the cassette section comprises a fender section configured to contact the external structure, the fender section comprising:a central protrusion; anda pair of recesses positioned at respective transverse sides of the central protrusion.

The recesses may comprise a stop surface, to prevent outboard motion of a first support tube of the external structure beyond the stop surface. The recesses and central protrusion may be configured so that contact between the first support tube and the stop surface causes rotation of the cassette section such that the fender section, suitably the central protrusion, contacts a second support tube of the external structure.

Further suitable features of the assembly of the fifth aspect are defined hereinabove in relation to the cassette assembly of the first aspect and vessel of the second aspect and may be combined in any combination.

According to a sixth aspect of the disclosure there is provided a cassette assembly attachable to an end portion of a waterborne vessel, the cassette assembly comprising:a cassette section to bear against an external structure in use, the cassette section movable relative to the vessel in a direction towards an opposite end of the vessel; anda resilient buffer system to counter movement of the cassette section when the vessel is moved towards the external structure,wherein the resilient buffer system comprises an over rotation prevention unit configured to prevent excessive rotation of the cassette section.

Further suitable features of the assembly of the sixth aspect are defined hereinabove in relation to the cassette assembly of the first aspect and vessel of the second aspect and may be combined in any combination.

In the drawings, corresponding reference characters indicate corresponding components. The skilled person 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 to improve understanding of various example embodiments. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various example embodiments.

DESCRIPTION OF EMBODIMENTS

In overview, examples of the disclosure provide a cassette assembly attachable to an end portion of a waterborne vessel, comprising a movable cassette section to bear against an external structure and a resilient buffer system to counter movement of the cassette section, wherein the resilient buffer system comprises at least two buffers spaced apart across a width of the cassette section, each buffer comprising a plurality of resiliently compressible elements arranged in series. The arrangement of a plurality of resiliently compressible elements in series may allow the buffer system to absorb more energy, whilst not exerting any significant increase in impact reaction force.

FIGS.1and2show the forward portion102of an example waterborne vessel100. The longitudinal centreline of the vessel100is indicated by reference numeral101. The forward portion102extends in a horizontal or transverse direction, such that it is generally perpendicular to the centreline101when viewed in plan.

The vessel100, particularly the forward portion102, comprises a cassette assembly, generally indicated by reference number200. As can be best seen inFIG.2, the cassette assembly200is disposed in a receiving portion120formed in the structure of the vessel100. For example, the receiving portion120takes the form of a recess extending in the transverse direction across the front portion102, configured to receive the cassette assembly200.

The receiving portion120is bound vertically by substantially horizontal deck plate of the vessel100. For example, the receiving portion120is bound by overhanging bulwark structure121above the receiving portion120and the hull122of the vessel100below.

Similarly, the receiving portion120is bound horizontally by at its transverse ends by substantially vertical bulkheads. In one example, the vessel100is a twin-hull vessel. In such an example, the substantially vertical bulkheads may be arranged on the centreline of each demi-hull of the vessel100. The rear of the recess120may be formed by a substantially vertical transverse bulkhead123.

The cassette assembly200comprises a cassette section210or carriage. The cassette section210takes the form of a platform, slidably mounted in the receiving portion120, so that it may move in the longitudinal direction indicated by arrow A. The cassette section210may also rotate in a horizontal plane, as indicated by arrow B. For example, the cassette section210may comprise arcuate sections at its transverse edges, supported by corresponding arcuate channels.

The upper surface211of the cassette section210allows users to access the water-accessible structure. For example, a walkway220may be formed on the upper surface211, though it will be understood that in some examples the whole of the upper surface211may be walked upon.

A front portion212of the cassette section210is configured to contact the water-accessible structure. The front portion212(and therefore the receiving portion120) may extend approximately 65-70% of the width of the forward portion102. As shown inFIG.1, the front portion212may comprise a fender section230. The fender section230is formed of a flexible or resilient material which may compress on contact with the water-accessible structure, such as rubber. The shape of the fender section230will be discussed in more detail with reference toFIGS.4and5hereinbelow. In some examples, the vessel100may also comprise fender sections105disposed at either side of the cassette assembly.

The cassette assembly200also comprises a resilient buffer system250. The resilient buffer system250is configured to counter movement of the cassette section210caused by contact between the cassette section210and the water-accessible structure. The resilient buffer system250comprises a pair of buffers251. The buffers251are spaced apart across the width of the cassette section210.

The buffers251extend from the rear bulkhead123of the receiving portion120to a stop surface213of the cassette section210. The stop surface213may be a substantially vertical surface, forming the end of a guide channel214extending horizontally into the cassette section210. The buffers251may be mounted to the rear bulkhead123by virtue of a mounting post252extending horizontally from the rear bulkhead, to which the buffers251may be secured.

Each buffer251comprises a plurality of resiliently compressible elements253. The compressible elements253are arranged in series in the direction A. The compressible elements253are substantially tubular compressible members with facing plates disposed at either end of the tubular member. For example, the compressible elements253take the form of rubber fender cells or cones. The compressible elements253are capable of being compressed to absorb the energy of the contact between the cassette section210and the water-accessible structure. Once the compression force is removed, the compressible elements253are configured to experience a restoring force, thereby returning to their initial, substantially uncompressed state.

In the example shown, each buffer251comprises 3 resiliently compressible elements253. In further examples, each buffer251may comprise 2, 4, 5, 6 or another number of resiliently compressible elements251.

In one example, the compressible elements comprise cell fenders. In one example, each compressible element has a longitudinal length of approximately 500 mm. The diameter of the tubular portion of the compressible element may be approximately 550 mm. The diameter of the facing plates may be approximately 650 mm. The compressible element may have a maximum deflection or compression of approximately 52.5%. Compression of the compressible element may require approximately 26 kNm energy, and provide a reaction force of approximately 120 kN. This may be rated performance data in accordance with PIANC, with an initial high-speed berthing velocity of 0.15 m/s. For example, the performance may be at 23° C.+/−5° C., at a compression angle of 0°. In one example, the compressible elements are SCK Cell Fenders provided by Trelleborg Marine Systems, suitably SCK 500 E1.3 cell fenders.

Guide projections255, which are shown in detail inFIG.3A-C, are mounted between neighbouring compressible elements253. The guide projections255project radially from the compressible elements253, for example in a vertical plane above and below the compressible elements253. The guide projections255are configured to interact with the guide channel214, so as to control the movement of the buffer251. For example, the guide projections255cooperate with corresponding slots255adefined in the channel214. The slots255amay be sized to prevent the buffer251from moving vertically. Accordingly, the depth of each slot255amay be substantially the same as the height of the projections255. In one example, each projection255comprises a nylon shoe, so as to reduce friction between the projection255and the slot255a. The guide projections255and corresponding slots255amay form a first or vertical guide for the buffer251, preventing motion in a vertical direction.

A second guide for the buffer251may limit the transverse range of motion of the buffer251. For example, the transverse width of the channel214may provide the limit to the possible transverse motion of the buffer251. In one example, buffer251and/or the channel214comprise low-friction elements that facilitate contact between the buffer251and the transverse edges of the channel. For example, the channel214may comprise nylon blocks257a, which are arranged to be contacted by stainless steel blocks257formed on the transverse sides of the buffer, for example mounted between neighbouring compressible elements253. The slot255amay also have a width that is wider than the projections255a, so as to permit transverse motion of the buffer251. Accordingly, the second guide limits transverse movement of the buffer251in use, so as to prevent shearing, whilst still allowing longitudinal motion and enough transverse motion to permit horizontal rotation of the cassette section210.

The resilient buffer system250furthermore comprises stop elements258, configured to limit the motion of the cassette section210in the longitudinal direction A, towards the opposite end of the vessel from the forward portion102. The stop elements258are disposed in the receiving portion120, on rear bulkhead123. For example, two stop elements258may be present for each buffer251, respectively disposed above and below the mounting post252. Accordingly, the stop elements258are configured to contact the cassette section210at regions above and below the channel215. The stop elements258are formed of a flexible or resilient material, which may deform upon contact. For example, the stop elements258may be rubber fender blocks.

The stop elements258ensure that, if the cassette section210is moved such that it would contact the rear bulkhead252, the impact is cushioned so as to prevent damage to the cassette section210and/or vessel100. In one example, the cassette assembly200is configured so that the cassette section210contacts the stop elements258before the buffer251reaches its maximum compression limit. Accordingly, damage to the buffer251is prevented and the life of the compressible elements253may be extended.

Turning now toFIG.4, the shape of the fender section230will be discussed in more detail. The fender section230comprises a central protrusion231, protruding longitudinally from substantially the transverse centre of the fender section230. The fender section also comprises two transverse protrusions232aand232b, respectively arranged port and starboard of the central protrusion231. Disposed between the central protrusion231and the transverse protrusions232a,bare port and starboard recesses233a,b.

Each recess233has a first sloping portion234, second sloping portion235and third sloping portion236, arranged consecutively extending away from the central protrusion231to its respective transverse protrusion232. The first sloping portion234slopes longitudinally inward (i.e. away from the front portion102) as it extends transversely away from the central protrusion231, and the second sloping portion235also slopes longitudinally inward as it extends away from the central protrusion231. The angle of slope of the second sloping portion235is shallower than the first sloping portion234. The recess233further comprises a third sloping portion236, which slopes longitudinally outward (i.e. towards the front portion102) as it extends towards the transverse protrusion. The angle a between the first and second sloping portions234,235is approximately 130 degrees. The angle β between the second and third sloping portions235,236is approximately 130 degrees.

Use of the vessel100and cassette assembly200will now be described with reference toFIGS.5A-C, which additionally show a water-accessible structure300.

For example, the water-accessible structure may be an offshore wind turbine300. The offshore wind turbine300comprises an access structure310formed on one side thereof, which allows access to the turbine300. The access structure310comprises a pair of spaced apart support tubes311, and a ladder312disposed between the support tubes311.

In use, the vessel approaches the turbine300, and is navigated so that the fender section230of the cassette assembly200impacts the support tubes311. Upon the fender section230contacting the support tubes311, a reaction force is imparted to the cassette section210. The reaction force causes the cassette section210to be urged in a rearward direction. The resilient buffer system250absorbs the shock of the impact by virtue of the compression of the buffers251, thereby preventing damage to the vessel100and/or the turbine300.

When the vessel100is in contact with the turbine300, the vessel100may be maintained in position by applying a forward thrust (e.g. via a propeller or other motive arrangement of the vessel100). Personnel wishing to access the turbine300may then step onto the walkway220, and onto the ladder312. Once any maintenance work has been completed and the personnel have returned to the vessel100, the vessel100applies a reverse thrust, thereby moving away from the wind turbine300. In doing so, the force which had compressed the buffers251is removed, and their resilient nature allows them to re-expand to the condition shown inFIG.2.

InFIG.5A, the vessel100is shown in aligned impact with the turbine300, such that the longitudinal axis101of the vessel100is perpendicular to a notional line extending between the tubes311. In this state, the central protrusion231is received in between the tubes311, with the tubes311contacting the recesses233a,bat either side of the central protrusion231. This is the ideal situation, with the impact force substantially aligned to the direction of compression of the buffers251. Furthermore, the location of the central protrusion231between the tubes311may assist in retaining the vessel100in location, for example in the event of wind or current acting on the vessel100.

FIG.5Bshows initial contact between the vessel100and the turbine300, with the vessel100approximately 10 degrees out of alignment with respect to the position shown inFIG.5A. In this misaligned approach, a first tube of the support tubes311is engaged by the port recess233a, and captured at the junction of the second sloping portion235aand third sloping portion236a. Accordingly, the second and third sloping portion235,236act as a stop surface, preventing further outboard motion of the first tube311with respect to the fender section230.

As can be seen inFIG.5C, the impact causes the port side of the cassette section210to move inwardly, causing rotation of the cassette section210in the horizontal plane. This brings the central protrusion231into contact with the second tube of the support tubes311. Accordingly, the shape of the fender section230ensures that both tubes are contacted, even in the event of a misaligned impact. This ensures the impact is shared between both tubes311, to prevent damage thereto. It also assists in ensuring that both resilient buffers251absorb the impact.

FIG.6A and6Bshows another example cassette assembly1200. The cassette assembly1200is similar and structure and operation to the cassette assembly200, with corresponding elements having corresponding reference numerals incremented by1000.

The cassette assembly1200further includes an over rotation prevention unit1260. The over rotation prevention unit1260comprises a plurality of restraining ties1261, extending from the stop surface1213to the mounting post1252. For example, there may be at least two restraining ties, disposed proximate the respective transverse ends of the stop surface1213. The restraining ties1261are configured to reach tension as shown inFIG.6Aand prevent the stop surface1213from moving further than the length of the tie1261away from the mounting post1252. The length of the tie1261is selected so that the stop surface1213cannot move to a position in which the resilient buffer1251is stretched beyond its uncompressed state. However, upon compression of the resilient buffer1251, the tie1261reverts to a relaxed state. The over rotation prevention unit1260therefore is able to prevent a strong contact at one end of the cassette section1210causing an excessive rotation of the cassette section210, thereby damaging the buffer1251the other end of the cassette section.

Although the resilient buffer1251of cassette assembly1200has only one resiliently compressible element1253, it will be understood that the over rotation prevention unit1260may be incorporated into the cassette assembly200.

It will be appreciated that, in some examples, the cassette assembly200or1200may be retrofitted to a vessel100having a suitable receiving portion. For example, the cassette assembly200or1200could be bolted or welded to the foredeck of the vessel, for example utilising the open space typically situated on the forward cargo deck of a vessel. Equally, the cassette assembly200or1200may be attachable to and detachable from the vessel100, for example for servicing, repair and the like.

In some examples, the cassette assembly may comprise a buffer comprising a plurality of compressible elements arranged in series, and a buffer comprising a single compressible element. The disclosure extends to any combination of any number of buffers comprising single or multiple compressible elements.

FIG.7illustrates a method of servicing a water-accessible structure using a waterborne vessel comprising a cassette assembly arranged at an end portion of the waterborne vessel. In block S701, a cassette section of the cassette assembly is brought into contact with the water accessible structure. In block S702, relative movement of the cassette section in a direction towards an opposite end of the vessel is countered using a resilient buffer system.

Advantageously, the above-described examples provide an improved vessel for accessing structures in water. The use of buffers comprising a plurality of resilient elements arranged in series provides a system able to absorb more energy whilst not exerting a significant increase in impact reaction force. Accordingly, vessels having a greater mass may be used to service water-accessible structures, and a larger tolerance of impact velocities may be permitted. Furthermore, the above-described examples provide a vessel which is able to approach the structure from a wider range of angles without causing damage to either the vessel or the structure, thereby easing operation of the vessel.