The present invention relates in particular to a method of employing a vessel in at least one process of repairing, maintaining, assembling, or disassembling an offshore installation or infrastructure, such as a windmill for generating electrical power or related infrastructure, and a related vessel, and assembly, Background

In marine offshore contexts, the provision of services at an offshore site can be challenging and difficult. Sites that are remote from land can be exposed to harsh environmental conditions, which may increase the challenges. For instance, when installing or maintaining offshore infrastructure such as a wind park or the like, services in the form of installation equipment, tools, build sections, and personnel may be required on site at various times and in various combinations in order to achieve objectives.

Particular difficulties have been identified by the inventors with regard to the provision of services for completing a process for instance at an offshore windmill installation site or similar. Such a "process" may typically include multiple operations, steps, or tasks performed as part of the process. It is also known to be difficult to transfer personnel and/or maintenance tools, equipment, parts, lubrication oils, etc., onto an installation for example to perform manual service, maintenance, or repair tasks. If services such as heavy lift cranes or similar equipment are needed repeatedly at the site, one solution has been to provide a jack-up rig adjacent to the site/installation. However, a drawback of the jack-up rig is that the legs of the jack-up rig are lowered onto the seabed in order to install the rig at the site. The engagement of the legs against the seabed typically damages the seabed in proximity to the installation site, such that it is typically not feasible for this reason to apply a jack-up at the site repeatedly more than on a couple or so occasions. In addition, the jack-up rig may not be used in deep water where the legs of the jack-up cannot reach the seabed.

Personnel access has otherwise typically been performed by use of small boats. Such a boat is typically run to the foot of the installation, and personnel then disembark from the boat onto the structure. In the case of windmill towers for instance, the personnel may then need to climb outside steps or a ladder from close to the sea surface upward to a work area or platform in an elevated position along the windmill tower. This can be hazardous, particularly in inclement weather. To address this, it has more recently been sought to utilise large ships in combination with a motion compensated gangway and/or crane in order to provide access for transferring personnel and/or equipment onto a platform or work area on the windmill tower higher up, but such ships also suffer from problems of motion characteristics in high sea states. Indeed, vessels of these kinds may typically heave, roll, and pitch with the waves such that if the waves are substantial in height, it can be difficult or completely unfeasible for personnel, tools, equipment, etc., to land on the installed structure. This can result in hazardous operations, operational delays, and expense.

Typically, many different kinds of vessels are employed, each for performing a specific operation in the process, and this can be associated with inefficiencies and costs.

The patent publication US2008/0135233 describes a modular installation vessel comprising first and second pontoon barges and interconnecting barges.

It is an object of the invention to obviate or at least mitigate various drawbacks or difficulties of prior art techniques. Summary

In light of the above, according to a first aspect of the invention there is provided a method of employing a vessel in at least one process of repairing, maintaining, assembling, or disassembling an offshore installation or infrastructure, the method comprising the steps of:

(a) providing a vessel, the vessel being provided with positioning means, wherein the vessel can perform a first repertoire of operations using equipment on the vessel;

(b) providing at least one attachable utility unit, the attachable utility unit comprising a hull and equipment supported thereupon;

(c) attaching the vessel to the attachable utility unit, so as to produce a combined assembly which is manoeuvrable by use of the positioning means of the vessel, wherein the combined assembly can perform a second repertoire of operations using either or both of the equipment on the vessel and the equipment on the attached utility unit; and

(d) moving the combined assembly to a site, using the combined assembly to perform at least one operation in said process, the operation in said process being one of the second repertoire of operations. in particular embodiments, the offshore installation may be an offshore windmill for generating electrical power. The windmill may be a floating offshore windmill for generating electrical power.

The vessel may preferably comprise a broad submerged hull and at least one narrow tower extending upward from the submerged hull such that the tower penetrates through the water surface. The positioning means may preferably comprise any one or more of: a dynamic positioning system;

propellers; and a depth and/or tilt positioning system. When the positioning means includes a dynamic positioning system, the positioning means of the vessel may further comprise propellers which are arranged to operate under control from the dynamic positioning system. The vessel alone can perform the first repertoire of operations. That is, the vessel can perform the first repertoire by itself, without any of the attachable utility unit(s) attached. The first repertoire of operations may typically comprise or consist of any of: transfer of personnel; lifting and/or handling operations; and transfer of light cargo or parts. in general, the method may include positioning the vessel using the positioning means or may further comprise positioning the combined assembly using the positioning means of the vessel. Such positioning of the vessel or the combined assembly may include any of:

moving or relocating the vessel and/or combined assembly by travel across the sea (e.g. under propulsion from propellers or other propulsion means), e.g. to and/or from the site of step d above, or to and/or from a site at which the vessel may be attached to the attachable utility unit; dynamically positioning the vessel and/or the combined assembly to hold a position or heading;

vertical positioning, whereby the vessel and/or combined assembly may be lowered or raised in the water; or

tilt positioning, where the vessel and/or the combined assembly may be tilted. in particular, the method may further comprise positioning the vessel with respect to the attachable utility unit using the positioning means. Such positioning of the vessel may be useful to correctly position the vessel to achieve subsequent attachment of the attachable utility unit. When

appropriately positioned, the vessel may be attached to the attachable utility unit, as set out in step c above.

The vessel may preferably be attached to the attachable utility unit by operating an attachment system provided on the vessel. The attachment system may preferably be remote-operated, and may preferably provide for quick-attach/release connection between the vessel and the attachable utility unit. Using the combined assembly in step d above may comprise using the equipment on either or both the vessel and the attached utility unit of the combined assembly to perform the operation in the process. The second repertoire of operations may comprise or consist of any one or more of:

handling or lifting parts, cargo, or containers; laying or repairing seafioor cable; and holding or positioning elements or sections of the offshore installation or associated infrastructure. The operation performed in step d above may be from the group comprising or consisting of: handling or lifting parts, cargo, or containers; laying or repairing seafioor cable; and holding or positioning elements or sections of the offshore installation or associated infrastructure

At least one operation of the second repertoire and/or the operation performed in step d above may typically be one which is outside of the first repertoire of operations and/or one which the vessel may not be capable of performing without the attachable utility unit or any attachable utility unit attached. The site of step d above may be a site at which the vessel and/or combined assembly is located adjacent to the offshore instailation, so as to allow the offshore installation to be reached by lifting and handling equipment, The method may further comprise, prior to step c above, using the vessel to perform a first operation to assist in the process of repairing, maintaining, assembling, or disassembling an offshore installation or infrastructure, the first operation being one from the first repertoire. The operation from the second repertoire performed by the combined assembly in step d above may be a second operation to assist in the process of repairing, maintaining, assembling, or disassembling an offshore installation or infrastructure. Typically, the first operation may be performed without any attachable utility unit or units attached.

The equipment of the attachable utility unit or respective attachable utility units may be selected from the group comprising or consisting of:

cable-laying or repair equipment;

at least one crane or jack;

at least one foundation support;

at least one service container;

at least one section of a tower of the windmill;

at least one blade for the windmill: and

at least one turbine, gearbox, or nacelle part.

The equipment of the vessel may comprise or consist of either or both of: at least one gangway, and at least one crane. Either or both of the gangway and the crane may be heave compensated.

The step of positioning of the vessel with respect to the attachable utility unit using the positioning means may include arranging a submerged portion of the vessel underneath part of the hull of the attachable utility unit. The vessel may be attached to the utility unit in step c above by forming a connection, e.g. an underwater connection, between the vessel and the attachable utility unit. More specifically, the connection may be formed between a submerged portion of the vessel and part of an underside, or side, of the hull of the attachable utility unit. The connection may be formed in step c with the underside or side of the hull of the attachable utility unit being arranged under the water.

The submerged portion of the vessel may typically be the submerged hull of the vessel or a part of the submerged hull. The submerged portion of the vessel may comprise a submerged deck of the vessel or part of the submerged deck. The submerged deck or part thereof may be substantially fiat, in particular embodiments, the connection may thus be formed between at least part of the submerged deck or hull of the vessel and at least part of the underside of the hull of the attachable utility unit. The step of positioning of the vessel with respect to the attachable utility unit using the positioning means may include lowering the vessel in the water and moving the lowered vessel laterally to allow said submerged portion of the vessel to be positioned underneath or on the side of the hull of the attachable utility unit.

The connection may be formed in response to activating the attachment system or one or more components or parts thereof.

The vessel may be attached via the attachment system by applying suction to a sealed region, by a magnet or magnetic force, or by other connecting means between the vessel and the attachable utility unit, to form the connection therebetween. The sealed region may be obtained between the submerged portion of the vessel and part of a side or underside of the attachable utility unit.

The attachment system may comprise one or more components or parts for forming the connection, between the vessel and the attachable utility unit. The attachment system may be operable to form the connection between the attachable utility unit and the vessel.

By way of the connection obtained, the attachable utility unit and the vessel may be held in place in fixed relationship relative to one another. The attachment system may comprise one or more components for forming the sealed region between the attachable utility unit (e.g. the part of the side or underside thereof) and the vessel (e.g. the submerged portion) for applying suction in the sealed region for forming the connection. Such components may be arranged between the submerged portion of the vessel and part of a side or underside of the attachable utility unit.

The attachment system may comprise one or more quick-attach connectors e.g. a suction connector, e.g. a suction cup or the like, e.g. operable to connect between the submerged portion of the vessel and part of the side or underside of the attachable utility unit for forming the connection. The attachment system may comprise at least one magnet or one or more components for generating a magnetic force between the attachable utility unit and the vessel, e.g. between the submerged portion of the vessel and part of the side or underside of the attachable utility unit, for forming the connection therebetween.

The method may include activating the attachment system to operate to form the connection between the attachable utility unit and the vessel, e.g. by an activation signal. When active, the attachment system may operate automatically to form the connection to attach the vessel to the attachable utility unit. The attaching step and/or activation of the attachment system may be initiated from a control computer or personnel-operable panel remote from the connection point, e.g. from a control room of the vessel. Accordingly, the attachment system may be remote operated. The instruction or signal to operate the attachment system to form the connection, e.g. data signal by electronic or other communication means, may be produced by vessel systems, e.g. when the vessel is positioned appropriately with respect to the attachable utility unit.

The vessel may typically be urged toward the attachable utility unit through operation of the attachment system and/or when in the combined assembly. By operating the attachment system, the connection formed may comprise a "kiss" or "cup" contact against the attachable utility unit, e.g. the underside or side of the hull of the attachable utility unit. The connection formed may comprise a pad contact connection, or a surface-to-surface contact connection, between the attachable utility unit and the vessel. The attachment system may include a pad or fender for producing frictionai resistance between the vessel and the attachable utility unit. The pad or the fender may be disposed within the sealed region.

The positioning means may further comprise any of; a navigation system; a steering system; and related parts.

The vessel may comprise at least two towers extending above the water surface from the hull.

The vessel may preferably be configured to have its centre of gravity below its centre of buoyancy when without any attachable utility unit or units attached, in some embodiments, the combined assembly (when the x'essel has at least one attachable utility unit attached) may have its centre of gravity below its centre of buoyancy.

The equipment of the attachable utility unit may include at least one lifting and/or handling device such as crane or jack and the operation may comprise a lifting and/or handling operation such as a lift, and the method may further comprise performing the lifting and handling operation using the lifting and/or handling device, e.g. performing the lift using the crane or jack.

The method may further comprise selecting the attachable utility unit or units to which the vessel is to be attached amongst a plurality of available floating attachable utility units each comprising a hull and equipment supported thereupon, wherein the selection may be performed based on a requirement of the operation or a requirement of the process in which the operation is to be performed.

The method may further comprise operating the vessel and/or the combined assembly to transfer personnel and/or small equipment between an offshore or onshore personnel accommodation facility and the offshore installation or infrastructure. The tower of the vessel may be provided with a gangway. The operating step to transfer personnel and/or small equipment may comprise any of: moving the vessel and/or the combined assembly adjacent to the accommodation facility and allowing personnel and/or small equipment to cross the gangway onto the vessel; moving the vessel and/or combined assembly onto site adjacent to the offshore installation or infrastructure; and allowing the personnel and/or smail equipment to cross the gangway onto the installation or infrastructure, the gangway being arranged on the tower of the vessei so that the gangway is elevated above the surface of the sea and an end of the gangway is positioned at a personnel platform on the offshore installation or infrastructure.

The step of operating the vessel to transfer personnel and/or small equipment may be performed prior to or without any attachable utility unit or units being attached to the vessei.

The method may further comprise detaching the connected attachable utility unit or units from the vessel. This may decrease a sensitivity of motion of the vessel and/or the combined assembly to environmental forces. The method may further comprise operating the vessel to transfer cargo or equipment off or onto the vessei and/or the combined assembly. The vessel may comprise a tower extending through a surface of the sea from a submerged hull, and the tower may be provided with a crane for cargo or equipment transfer. The operating step may comprise any of: moving the vessel and/or the combined assembly adjacent to the offshore installation or infrastructure or another offshore platform; and transferring the cargo or equipment using the crane, the crane being arranged on the tower so as to be elevated above the surface of the sea and reach across between the tower of the vessel and the offshore installation or other offshore platform. The step of operating the vessel to transfer the cargo or equipment may be performed prior to or without any of attachable utility unit or units being attached to the vessel,

The vessel and/or combined assembly may be positioned to keep a lateral position and/or a heading. The positioning of the vessel and/or the combined assembly may be performed by using the dynamic positioning system and propellers. The positioning may include dynamic positioning at a destination site.

The vessei and/or combined assembly may be positioned to keep a lateral position and/or a heading of the vessel with respect to the attachabie utility unit to be attached, in order to attach or when attaching the vessei to the attachabie utility unit. The vessel and/or the combined assembly may be positioned to keep lateral position and/or heading of the vessei adjacent to the offshore installation, in order to perform or when performing the operation in step d above, or when performing any other operation using the equipment of the vessei, attachabie utility unit, and/or the combined assembly, e.g. the first or second operations, or the transfer of small equipment or parts between the installation and the vessel and/or combined assembly. The vessel and/or the combined assembly may include a coupling system operable to attach the vessel and/or the combined assembly to the offshore installation. The coupling system may be remote operable.

The method may include, after moving the vessel to the site, attaching the vessel to the offshore installation. The attachment may be performed by operating the coupling system.

The vessel and/or the combined assembly may be positioned to keep lateral position and/or heading of the vessel with respect to the offshore installation in order to attach the vessel and/or the combined assembly to the offshore installation. The operation in the process in step d may be performed when the vessel is attached to the offshore installation. When attached to the offshore installation, the vessel may be restricted or substantially prevented from movement in at least one direction with respect to offshore installation.

The vessel may be positioned by lowering, raising, or levelling out at even keel in the water the vessel, e.g. in a first configuration of the vessel where the attachable utility unit or units are not attached to the vessel, e.g. prior to attachment to the attachable utility unit(s). The combined assembly may be positioned by lowering, raising or levelling out at even keel the combined assembly, e.g. in a second configuration of the vessel where one or more of the attachable utility units are attached.

In general, the lowering, raising, or levelling out at even keel the vessel and/or the combined assembly may be performed by operating the depth and/or tilt positioning system. The depth and/or tilt positioning system may typically comprise at least one ballast and/or trim tank which may be arranged to be provided with ballast. in order to position the vessel with respect to the attachable utility unit to allow attachment of the vessel to the attachable utility unit, the depth and/or tilt positioning system of the vessel may be operated to help to position the vessel with respect to the hull of the attachable utility unit. In particular, the depth and/or tilt positioning system may be operated to move the submerged hull of the vessel upward so as to bring a part of the submerged hull toward a bottom of the hull of the floating attachable utility unit for connection to an underside of the hull of the attachable utility unit. in embodiments where the hull of the attachable utility unit comprises at least one leg extending between a keel and a deck, the lowering, raising, or levelling out at even keel the combined assembly may be performed to position the combined assembly so that the hull of the attachable utility unit may be arranged to intersect the water surface at a location along the leg. When the combined assembly is so arranged, the operation in step d using the equipment on the vessel and/or the attachable utility unit may be performed. The lowering, raising, or levelling out at even keel the combined assembly and/or the vessel may be performed to position the gangway and/or crane on the vessel so that the crane or gangway is arranged at a desired height above the surface of the water or at a desired height along the offshore installation, e.g. a tower of an offshore windmill, for example so that the gangway is level with a platform on the tower of the windmill. The desired height may be a height providing a safe distance above an expected wave height, to facilitate transfer of personnel and/or small equipment between the vessel and the offshore installation. The vessel may be raised or lowered by an amount in the range of up to 20-30 m vertically

The attachable utility unit to be attached to the vessel, or any one or more of a plurality of available utility units from which the attachable utility unit to be attached is selected, may be towed to at least one collection location.

The method may further include moving the vessel to travel to the collection location to attach to the attachable utility unit. The floating attachable unit may be non-propelled. Typically, the attachable utility unit may comprise a barge, e.g. typically a flat-bottomed barge. At least some of the equipment on the attachable utility unit may be operated using fluid and/or electrical power supplied to the attachable unit from the vessel when attached

According to a second aspect of the invention, there is provided a vessel configured to be employed in at least one process for repairing, maintaining, assembling, or disassembling an offshore installation or infrastructure, the vessel having positioning means and being configured to attach to at least one attachable utility unit, for forming a combined assembly which is manoeuvrable by the positioning means, for performing at least one operation in said process.

The vessel may preferably comprise a broad hull or section thereof arranged to be submerged under water, and at least one narrow tower arranged to extend upward from the hull or section thereof when submerged such that the tower penetrates through the water surface. The positioning means may preferably comprise any one or more of: a dynamic positioning system; propellers; and a depth and/or tilt positioning system. When the positioning means includes a dynamic positioning system, the positioning means of the vessel may further comprise propellers which are arranged to operate under control from the dynamic positioning system.

The vessel may typically further comprise an attachment system for attachment of the vessel to the attachable utility unit, for forming the combined assembly. The attachment system may preferably be remote-operated, and may preferably provide for quick-attach/release connection between the vessel and the attachable utility unit. The vessel may typically be operable in a first configuration without an attached utiiity unit to perform a first repertoire of operations using equipment on the vessel. The first repertoire of operations may typically include either or both of: transfer of personnel; and transfer of light cargo or parts.

The vessel may typically be operable in a second configuration in the combined assembly. The combined assembly may be operable to perform a second repertoire of operations using either or both of the equipment on the vessel and the equipment on the attached utiiity unit, for performing the operation in said process. The operation performed in said process may typically be one of the second repertoire, and may typically be from the group comprising: handling or lifting parts, cargo, or containers; laying or repairing seafloor cable; and holding or positioning elements or sections of the offshore installation or associated infrastructure The vessel alone may be capable of performing the first repertoire of operations. That is, the vessel may be capable of performing the first repertoire by itself, without any of the attachable utility unit(s) attached.

The offshore installation may be an offshore windmill for generating electrical power. The windmill may be a floating offshore windmill for generating electrical power. The offshore installation may be a platform associated with an offshore wind park: or may be another offshore installation such as an unmanned oil and gas installation or platform.

The vessel may have further features as described in relation to the first aspect above, the claims, or elsewhere herein.

According to a third aspect of the invention there is provided an assembly comprising the vessel according to the second aspect and at least one attachable utility unit being attached to the vessel. According to another aspect, there is provided a method of employing a vessel in at least one process of repairing, maintaining, assembling, or disassembling an offshore installation or Infrastructure, the method comprising the steps of:

(a) providing a vessel, the vessei being provided with positioning means;

(b) providing at least one attachable utility unit, the attachable utiiity unit comprising a hull and equipment supported thereupon;

(c) attaching the vessel to the attachable utility unit, so as to produce a combined assembly which is manoeuvrable by use of the positioning means of the vessei; and

(d) moving the combined assembly to a site, and at the site, using the equipment of the attachable utility unit in the combined assembly to perform at least one operation in said process. In this way, by attaching the vessel to the attachable utility unit, a repertoire of operations which may be performed to facilitate in the process may be expanded. Any of the aspects above or elsewhere may include further features as described in relation to any other aspect, wherever described herein. Features described in one embodiment may be combined in other embodiments. For example, a selected feature from a first embodiment that is compatible with the arrangement in a second embodiment may be employed, e.g. as an additional, alternative or optional feature, e.g. inserted or exchanged for a similar or like feature, in the second embodiment to perform (in the second embodiment) in the same or corresponding manner as it does in the first embodiment.

Embodiments of the invention can be advantageous in various ways, in various embodiments, the vessel may have a favourable, low motion response to the weather elements in particular when without any attachable utility unit being attached The vessel may advantageously be able to be positioned in various embodiments by dynamic positioning and by raising or lowering the vessel in the water, which may facilitate use of the vessel in difficult weather. The motion response and ability to position the vessel may facilitate and help to more easily arrange the vessel in position and attach the vessel to the attachable utility units. The attachment system may advantageously provide for obtaining a connection between the vessel and the attachable utility unit swiftly and reliably.

Furthermore, the second repertoire of operations may give a greater functional capability than achievable through the first repertoire of operations, such as when the vessel is not attached to any attachable utility units. Accordingly, the functionality provided may be expanded by the attachment of the vessel to one or more attachable utility units. The attachment of the attachable utility unit may thus add to and expand on the functionality of the vessel in its own right. Different attachable utility may be added or exchanged with others which may have equipment adapted to provide particular functionality and/or provide required specialist functions in order to repair, maintain, assemble, or disassemble the offshore installation or infrastructurer Description and drawing

There will now be described, by way of example only, embodiments of the invention with reference to the accompanying drawings, in which;

Figure 1 is a schematic representation of apparatus including a vessel according to an

embodiment of the invention, in use;

Figure 2 is a schematic representation of the apparatus of Figure 1 wherein the vessel is

arranged in a combined assembly with selected attachable utility units for performing an operation;

Figure 3 is a schematic representation of the vessel of Figure 1 alongside an offshore

installation for lifting operations and gangway access; and Figure 4 is a schematic representation of apparatus including a vessel according to another embodiment of the invention. Turning firstly to Figure 1 , there is shown apparatus 1 including a vessel 10 on the water 2. The vessel 10 is illustrated nearby a windmill 40, which is subject to maintenance or under construction, The windmill 40 is part of a wind park comprising typically several windmills and associated infrastructure such as seabed cabling and support equipment (not shown) as may be employed for example in converting and delivering produced electrical power from the windmill 40 to shore.

The vessel 10 is illustrated in the form of a base unit which can operate on its own to perform simple operations or tasks, such as crew transfer or light maintenance equipment or cargo transfer. As will be described, various attachable utility units can be attached to the vessel 10, more specifically to the base unit. In Figure 1 , the vessel 10 consists of only the base unit without any such utility unit attached, in embodiments of the invention, any reference to the vessel can be replaced by reference to the base unit.

By addition of an attachable utility unit to the vessel, the repertoire of operations which may be performed to assist in the process of maintenance of construction of the windmill may be expanded from a first repertoire when the vessel is operating without any such attachable utility unit attached to a second repertoire when the attachable utility unit is attached and combined with the vessel in a combined assembly which can travel together and be positioned under propulsion and the positioning means on the base unit vessel. Therefore, the functionality that can be provided by using the vessel to attach and bring the combined assembly of the vessel and the attachable utility unit to the site of the windmill, can advantageously be changed and/or adapted as needed by the addition of the attachable utility unit. Speciality equipment on the attachable unit may be utilised to provide specific functions, and when no longer required one attachable utility unit can be changed out with another attachable utility unit which is fitted with other equipment that may assist to provide other desired functionality. In the example of Figure 1 , the apparatus 1 includes a group 30 of individual attachable utility units 31 - 35, which are each provided on the water 2 at the water surface as floating barges with equipment or construction blocks for facilitating in one or more operations on the wind park. More specifically, a first attachable utility unit 31 is provided with a crane 51 a or a jacking device 51 b for performing lifting/jacking operations. A second attachable utility unit 32 is provided with containers or cargo units 52. A third attachable utility unit 33 is configured to hold a section of an offshore structure to be installed, the section in this example being a foundation structure 53 for a windmill. The group of utility units further includes a fourth unit 34 carrying blade sections 54 or other windmill parts for the windmill 40 under construction or subject to maintenance, and a fifth unit 35 is provided with a cable device 55 for spooling out cable onto the seabed or for use in cable repair. By attachment of the vessel 10 to one or more of the attachable utility units, a combined assembly is provided which is configured appropriately for performing an operation. As can be seen in Figure 2 for instance, through attachment of the first attachable utility unit 31 to the vessel 10, the vessel 10 can be configured with a crane or jacking device and can thus be capable of performing high lifts such as for lifting e.g. a windmill blade section onto the windmill 40 under construction/maintenance. The vessel 10 in Figure 2 is additionally configured with the third attachable utility unit 33 attached, so that the crane/jacking unit can readily access the sections of e.g. the windmill blade. The vessel 10 is moved and steered to the required location on the wind park to perform the operation, e.g. high lift or other lifting and/or handling operation, using the utility units attached.

In practice, a range of floating attachable utility units 30 with equipment of different kinds may be available. The vessel 10 may be utilised with different combinations of the units 31 -35 based on different tasks to be performed. The attachable utility units 31-35 may be available at a collection location near shore or away from shore or in proximity to the wind park to provide increased convenience of access for one or more vessels 10 at one or several different wind parks. In the embodiments of Figures 1 and 2, the vessel 10 can take one or two such units.

It can be noted that the second unit 32 has a "split" hull, with columnar legs extending between a buoyancy element 32b and a keel portion 32k. The containers 52 are supported on a deck of the hull above the water line. When the utility unit 32 is attached to the base unit of the vessel, the unit 32 is positioned such that the legs pass through the sea surface 3. The small cross-sectional area of the legs can limit the effects of surface waves on the motion of the vessel. It can also be noted that in order to place the foundation 53 on the seabed 4 when the third unit 33 is attached to the vessel 10, this may be carried out by lowering the vessel 10 (and hence also lowering the attached third utility unit 33) in the water so that the foundation 53 is brought onto the seabed 4. In practice therefore, the foundation 53 is held from the third unit 33 typically quite close the seabed 4 so that the seabed 4 can be reached by lowering the vessel 10.

The vessel 10 can attach to an attachable utility unit 31 -35 through an attachment system 12 arranged on a laterally extending deck 1 1 of the vessel 10. The attachment system 12 is a quick-attach/release system, which may for example comprise a magnet connector, a suction connector or other means. In order to attach to one of the units 31-35, the vessel 10 is lowered so that the deck 1 1 is submerged and is moved laterally underneath the base of the floating utility unit to be attached, with the attachment system 12 positioned against the base. In embodiments using suction connection, the suction is applied to a sealed region between the submerged deck 1 1 and the base of the floating unit so as to fixedly attach the two securely together. Alternatively, it can also be arranged so that the attachment system is located on the side of the vessel for connection to the side of the attachable units. Typically, the attachable utility units are flat-based, i.e. the hulls have flat undersides, to allow for connectivity. The vessel 10 has a hull 15 including ballast/trim tanks 16 for ballasting the vessel 10 in the water. One or several towers 17 extend upright from the hull 15 and penetrate through the water surface. Typically, the tower 17 may comprise two columns connected by a bridge between the columns at upper ends of the columns. When deployed alone (without any utility units attached), the base unit vessel 10 has a centre of gravity below the centre of buoyancy. The hull 15 has broad lateral extent compared with the tower 17, such that with the hull submerged beneath the water surface 3, as seen in Figure 1 , only the tower penetrates the water surface. The low centre of gravity and low cross- sectional surface area of the tower or elements thereof at the water interface means that wave effects at the sea-surface only result in small amounts of roll, pitch, and heave motion being imparted to the vessel, allowing for operation in comparatively high sea states.

In general, the attachable utility units can be provided and moved to a convenient location or another location (e.g. another wind park) where there may be a need for the utility units. One or more of the utility units 31-35 may thus be moved elsewhere for use by other vessels and/or in other locations (e.g. ones configured in the same manner as the vessel 10) if the attachable units are not needed at a particular site, so that the attachable utility units can be better utilised. Thus, it will be appreciated that a given vessel 10 does not necessarily have at its disposal a full set of the functionality provided by the attachable utility units at all times. Indeed, an advantage of embodiments of the invention is that utilisation can be optimised so that the vessel does not need to be equipped with or carry or have immediate access to ail types of equipment at all times. This can also facilitate efficiencies in terms of limiting quantities of equipment aboard the vessel, vessel tonnage, mobility and control of the vessel, fuel efficiency, and costs.

The vessel 10 is provided with positioning means so that it can be moved and manoeuvred into appropriate position for attachment to the attachable utility units 31-35.

The ballast/trim tanks 16 in the hull 15 can be used to raise or lower the vessel 10 vertically in the water and/or to trim/tilt the vessel 10. The ballast/trim tanks may let in or out water for positioning the vessel 10 at the appropriate vertical position within the water, or to compensate the trim to offset weight onto the vessel 10. In this way, the vessel 10 may be lowered so that the deck 1 1 is submerged to allow it to be moved underneath the base (e.g. beneath an underside) of the attachable utility units and to allow the attachment system 12 to be positioned adjacent to the base (e.g. on the underside thereof) of the attachable utility units for connection to take place.

When the vessel 10 is used for an operation with one or more of the attachable utility units being attached, the hulls of the attachable utility units 31-35 are arranged at the water interface. The vessel 10 can be operated through the ballast/trim tanks to adjust vertical position and through propulsion/positioning means to move location and adjust lateral position and orientation. The attachable utility units 31 -35 are for instance anchored and/or held in place by a tug (not shown), although even if these are free to move, the vessel 10 can dock onto and attach to the attachable utility units 31 -35. The vessel 10 may automatically track the speed and directional orientation of the attachable utility units 31 -35, e.g. through a tracking system on the vessel, and gradually approach a given attachable utility unit to dock onto it.

The vessel 10 is propelled and steered by the propellers 18. The vessel 10 can thus be moved to any desired location or orientation for collecting and attaching to the floating attachable utility units and carrying them to a desired location e.g. for performing an operation on the wind park. The vessel 10 also includes a dynamic positioning system which is coupled to the propellers 18 and through which the propellers 18 can be controlled automatically to keep the vessel 10 in a selected desired position despite environmental forces, such as from wind, waves, or currents, which may tend to push the vessel away from the desired position.

The vessel 10 or any of the attachable utility units 31 -35 may be equipped with a coupling system (not shown in Figures 1 to 3) to connect the vessel to the windmill/installation 40. This coupling system may allow for freedom of movement and rotation in zero, one, or two directions.

The tower 17 includes a control room 19 and may include personnel quarters 20 in an upper part of the tower 17. The tower 17 is also provided with a gangway 21 for transfer of personnel and/or small equipment between the vessel 10 and an offshore installation such as the windmill 40 or another installation. In addition, the vessel 10 in this example has a small crane 22 for transfer of minor tools and parts. The gangway 21 and the small crane 22 may have heave compensation. In particular embodiments, the vessel 10 may be utilised to transport personnel and equipment from an offshore accommodation facility onto an installation on the wind park such as the windmill 40, e.g. for providing maintenance, service or inspection. Such operations can be performed without one or more, or any, of the attachable utility units 31 -35 attached.

In Figure 3, the vessel 10 is exemplified without any of the attachable utility units 31 -35 attached, in position alongside the tower of the windmill 40, where the gangway 21 extends between the tower 17 on the vessel 10 and an access area 42 on the windmili 40 for the personnel to walk over the gangway 21 between the vessel 10 and the access area 42. The location of the gangway 21 high up on the tower allows for direct access to the windmill deck area 42. The small motions of the vessel 10 due to its low centre of gravity, the tower 17 with small area extending through the water surface, and dynamic positioning, allows such access to take place safely in a wide range of weather conditions, without exposing personnel to risks in having to climb a windmill tower. The benefits of small motion can also apply to the small crane when performing lifts to transfer items to the windmill tower. As can be noted from Figures 1 to 3, the vessel 10 is a semi-submersible floating vessel which can be freely and readily steered and moved from one location to another across an area of the sea 2. The vessel 10 is not anchored or otherwise coupled to the seabed 4, but uses dynamic positioning, when required, to keep constant heading and position in the sea 2. An advantage of this is that the vessel 10 can be moved repeatedly to a specific location or site, e.g. adjacent to an offshore installation such as the windmill 4, to perform operations in harsh environments without disruption to the seabed 4. Furthermore, through its design, the vessel 10 as described above provides a solid and stable structure in the sea 2 with low motion characteristics in a range of conditions for supporting cranes or other equipment for performing an operation, as a base unit or in combination with the attachable utility units.

In use, the vessel 10 may be employed for transferring personnel between a permanent central accommodation facility and an installation on an offshore wind park. The personnel may be trained for performing service, maintenance, or inspection tasks on the installation. The vessel 10 is positioned adjacent to the accommodation facility, and personnel are transferred from the accommodation facility onto the vessel 10 via the gangway 21. In an alternative embodiment, an accommodation facility can be built into the vessel 10 itself.

From time to time, the vessel 10 is configured for performing specific operations on the wind park using floating attachable utility units 31 -35, which are attached to the vessel 10 and which include equipment such as tools or work parts as may be required for performing the operations. The floating attachable utility units each comprise a hull with equipment and/or parts supported upon the hull in the water. A number of attachable utility units 31 -35 can be arranged at one or more collection locations. The vessel 10 is moved to the collection location and the hull 15 of the vessel 10 is submerged so that the deck 11 is arranged beneath the underside of the base of the hull of a selected one of the attachable utility units and, by use of its attachment system 12, the vessel is connected thereto so as to attach the attachable utility unit to the vessel 10 in a combined assembly. The vessel 10 may then connect another one of the attachable utility units 31 -35 to another attachment system 12, in the same manner. With the selected ones of the attachable utility units 31 -35 attached, the vessel 10 is moved to a location where an operation is then performed using the attached utility units. A typical operation may be a lifting/jacking operation using a crane/jack of the attachable utility units to lift a section of a windmill into place at an installation location of the windmill. In certain embodiments, the vessel 10 can have the attachable utility unit 35 containing the cable reel 55 attached and may be used to lay cable on the seabed 4 by spooling out the cable from the reel 55 while travelling to a destination location, before thereafter performing another operation, such as performing a lift using a crane/jack, at the destination location. In some embodiments, the attachable utility unit 32 with containers 52 may be attached and the small crane 22 or a high-lift crane 51 a on another unit 31 attached to the vessel 10 may be used to manoeuvre one or more of the containers 52. The vessel 10 is controlled and steered to keep in position, e.g. by the propellers and the dynamic positioning system of the vessel, while performing the necessary operations. Once the operation has been completed, the vessel 10 may return to the collection location and the utility units may be disconnected. The attachable utility units 31-35 may be transferred to other windmill parks and/or may be used by another vessel substantially the same as the vessel 10. Other attachable utility units may later be attached to the vessel 10 according to requirements.

The connectivity of the vessel 10 is such that it allows tools and heavy equipment to be attached and/or released quickly and reliably through an attachable unit. Typically, this is achieved through the attachment system by a suction connection, whereby suction is applied between the base of the attachable utility unit and the submerged deck of the vessel. The attachable utility units may advantageously be prepared in advance, and may be towed to the collection location offshore. This may facilitate convenience, so that the vessel 10 does not have to travel far to access the attachable utility units. The attachable utility units offer flexibility in how the vessel 10 can be configured.

Servicing, maintenance, installation and repair activities can be coordinated and optimised. The number of attachable utility units can be minimised since they can be used by several vessels 10 and on several wind parks.

The ability to configure the vessel 10 readily and offshore for different types of operations can help to reduce the need to engage other vessels. Personnel and maintenance parts can be transferred to installations in a greater range of environmental conditions, increasing the ability to service or perform repairs and reducing risks in production downtime from wind parks.

In other embodiments (not shown), the hull 15 of the vessel 10 is adapted to carry a foundation for a windmill, e.g. via the utility unit 33, so that the foundation can be carried by the vessel 10 to the required location for installation and lowered into the water. In such embodiments, the hull may comprise a slot arranged to contain the foundation laterally between sides of the slot at the side of the hull. The hull may be lowered in the water to lower the foundation onto the seabed.

Other parts of a windmill, such as sections of a windmill tower, may be held at the side of the vessel in a similar manner, although in such cases, the vessel is not lowered to position the sections.

With further reference now to Figure 4, a vessel 110 is exemplified in use with a floating windmill 140. The vessel 1 10 has an attachable utility unit 135 attached. The attachable utility unit 136 is attached generally in the same manner as described above in relation to the vessel 10. The attachable utility unit 138 has a hull 136h which has vertical columns 136p extending between an upper part 136b and a base 136k of the hull 136h. Prior to attachment, the vessel 110 is typically operated without any attachable utility unit attached and the attachable utility unit 136 is arranged at an offshore location awaiting attachment. The vessel 136 sails to the offshore location and is raised up, lowered down, and/or moved laterally in the water as required to position the vessel in proper position and orientation with respect to the attachable utility unit 136, so that the hull 1 15 of the vessel 110 is in position underneath the base of the hull of the utility unit 136 and is ready for attachment. The vessel 1 10 may be moved upward so that the hull 115 comes into contact with the underside of the base 136k of the attachable utility unit with an upward force against the underside. The attachable utility unit 136 may thus come to rest upon the hull 115 of the vessel 110, allowing an attachment system to be activated to securely attach the attachable utility unit 136 to the vessel 110.

After attachment, the vessel 1 10 with the attachable utility unit 136 attached travels to the location of the windmill 140 as seen in Figure 4. The attachable utility unit 136 has no means of propulsion in its own right. In contrast, the vessel 1 10 has on board propulsion means so that the vessel 1 10 can in principle sail to any given destination. Hence, under propulsion and control of the vessel 110 the attachable utility unit 136 (when attached to the vessel 1 10 in a combined assembly) can be brought to a desired site of use, i.e. adjacent to the location of the floating windmill 140 in this example.

As seen in Figure 4, when the attachable utility unit 136 is attached, the base 136k of the hull 136h of the attachable utility unit 136 is supported upon the submerged hull 1 15 of the vessel 110. The attachable utility unit 136 is attached to the hull 1 15 by a quick attach/release attachment system, which is remote-operable to generate a connection between the base 136k of the hull 136h of the attachable utility unit 136 and the submerged hull 1 15 of the vessel.

In this example, the attachable utility unit 136 is equipped with a lifting and handling device 156 for lifting, lowering, and/or supporting a windmill blade 145. Figure 4 illustrates the lifting and handling device 156 supporting the windmill blade 145 at an elevated position such as is applicable in a process of installing the blade 145 onto the windmill 140.

The combined assembly of the vessel 1 10 and attachable utility unit 136 is also attached to the floating windmill 140 by a coupling system including first and second couplers 181 , 182. The first coupler 181 is mounted on the attachable utility unit 136, and provides a connection between a part of the attachable utility unit 136 and the tower of the floating windmill above the water surface 103. The second coupler 182 in this example is mounted on the submerged hull 1 15 of the vessel 1 10 and provides a connection between a part of the submerged hull 115 of the vessel and the tower of the floating windmill, beneath the water surface. In other variants, a coupler of the coupling system, e g. the second coupler 182 may be provided on the attachable utility unit 136 and be lowered down so as to be positioned in a similar location to that of the second coupler 182 in Figure 4, and be supported by the keel of the vessel.

The couplers 181 , 182 operate to lock the combined assembly (i.e. the vessel 1 10 and attachable utility unit 136) relative to the floating windmill 140. In general, the couplers 181 , 182 may be arranged to allow movement in one or more directions and may prevent movement in other directions. For example, the couplers 181 , 182 may restrict the combined assembly to have none, one, or two degrees of freedom of movement with respect to the windmill 140.

The couplers 181 , 182 operate by remote-activating the coupling system to generate the connection of the couplers 181 , 182 to the tower of the windmill 140. They coupling system may be activated to generate connection of the couplers 181 , 182 once the couplers 181 , 182 are positioned appropriately adjacent to the tower of the windmill. The vessel is operated by way of the propulsion means to place the couplers in position. The vessel 1 10 is equipped with dynamic positioning to facilitate positioning in appropriate lateral position adjacent to the windmill tower. The couplers may comprise suction pads, hydraulic g rippers or magnets which attach to the tower.

The vessel 1 10 is equipped with vertical positioning means 116 (e.g. ballast tanks). The vertical positioning means can be used when the attachable utility unit 136 is attached in the combined assembly to change the elevation of the vessel. This can be useful to position the equipment on the vessel or the lifting and handling means 156 on the attachable utility unit 136 to an appropriate position along the windmill. In this example, the vertical positioning means is used to position the combined assembly so that the hull 136h of the attachable utility unit is positioned with the vertical columns 136p intersecting the surface of the water 103. This can facilitate to provide favourable motion characteristics due to the use of columns 136p having a small area of intersection with the sea surface 103.

The configuration of the vessel 110 itself, like the vessel 10 described above, is also configured so as to have favourable motion characteristics, e.g. through the provision of a broad submerged hull 115 and keel 126 and penetration through the water surface 103 of a narrow tower 1 17. The vessel can then be highly stable and the effects of waves on the motions of the vessel can be a lot lower than with typical vessels. The positioning of the vessel 1 10 by the vertical positioning means and dynamic lateral positioning means can further facilitate positioning the vessel or combined assembly (vessel and attachable utility unit) favourably and accurately in varying or difficult sea conditions. As a result a greater uptime in operations such as for assembling, disassembling, maintaining, or repairing the windmill or associated infrastructure may be achievable through the present solution.

Various modifications and improvements may be made without departing from the scope of the invention herein described. While the above description is made specifically with reference to windmills, it will be appreciated that the invention could also be utilised in relation to other types of offshore structure, e.g. where particular kinds of operation and/or personnel access is required. The term "equipment" is used herein to include tools, construction elements, and other parts. It can be appreciated that the attachment of the vessel to the utility units beneficially can be achieved quickly and reieasabiy through the attachment system without any bolting, welding, or hands-on connection work taking place or otherwise being required.