GB2628092A

GB2628092A - Semi-submersible floating offshore wind vessel having a submersible hull comprising variable ballast pontoons

Info

Publication number: GB2628092A
Application number: GB2303468.9A
Authority: GB
Inventors: Stewart Whittle Martin Richard
Original assignee: Trivane Ltd
Current assignee: Trivane Ltd
Priority date: 2023-03-09
Filing date: 2023-03-09
Publication date: 2024-09-18
Anticipated expiration: 2043-03-09
Also published as: GB202303468D0; GB2628092B

Abstract

A floating structure (1, Fig 1) for supporting a horizontal axis wind turbine (13, Fig 1) has a submersible hull 3 to which three buoyant bodies 5, 7 and 9 are attached and a wind turbine tower support structure 11. It is assembled from two sub-assemblies 16 and 18 that are each independently buoyant and independently stable when floating on water. A pontoon 15 of the first sub-assembly is provided at its first end (39 Fig 3) with a first connecting feature of a connection arrangement (19, Fig 3) which is connected during assembly to a second connecting feature of the connection arrangement that is provided as part of the first side wall (65, Fig 5) of a pontoon 17 of the second sub-assembly. The first side wall of the second pontoon is located substantially within a single plane that is orientated during assembly to be perpendicular to the axis X-X of the first pontoon. The first and second pontoons each comprise ballast tanks (60, 62, Fig 3) and (88 ,90, Fig 5) so that, during assembly the heel of the first and second sub-assembly can be adjusted.

Description

SEMI-SUBMERSIBLE FLOATING OFFSHORE WIND VESSEL HAVING A SUBMERSIBLE HULL COMPRISING VARIABLE BALLAST PONTOONS

The present invention relates to floating offshore wind (FOVV) vessels for supporting a single wind turbine for the generation of electricity from wind. The floating vessel has a submersible hull supporting buoyant members and a wind turbine tower support. The submersible hull can be partially submerged during assembly on the water and during tow out to an installation site. The floating vessel is ballasted down when at the installation site, for example at an offshore wind turbine farm, so that the submersible hull is underwater whilst operating moored offshore.

io The buoyant columns and the wind turbine tower support remain at least partially above the water level at all times, such that the vessel as a whole is a semi-submersible. In a first embodiment the floating vessel is moored using a spread mooring. In a second embodiment the floating vessel is used in conjunction with a mooring system that permits the floating vessel to weathervane in order to head the vessel into the prevailing seas. In the second embodiment the buoyant members are ship-shaped hulls, which results in a reduction in the loads to which the mooring system is subjected. In the first embodiment and in the second embodiment the submersible hull and the buoyant members are made from stiffened flat plates, which means that these components are easier for many yards and fabrication shops to make, compared to the manufacture of cylindrical hulls, thus making them cheaper.

The mounting structure of a conventional offshore wind turbine is referred to in the wind industry as a tower. Such a tower is a vertical hollow column of varying cross-sectional profile, sometimes with a lattice structure beneath it. Such mounting structures are normally installed in water depths of up to thirty metres and occasionally more. It is presently considered that the theoretical and practical limit for installing these fixed structures is a water depth of around fifty metres. If the water depth is greater than around thirty metres then a floating structure can be used to support the wind turbine. Another consideration is the suitability of the nature of the seabed. The tower of a fixed wind turbine is normally mounted on a large pile driven into the seabed, but in some locations the tower needs to be inserted into a socket that is bored into rock, or else mounted on a concrete gravity base, either solution being costly. On the other hand, some floating structures, including the floating vessel of this invention, are restrained by mooring lines that are fixed to the seabed using embedment anchors, or suction anchors, or driven piles that only carry horizontal load. These are less costly to provide and install, compared to a gravity base or to boring a socket to receive a tower of large diameter, and so this consideration, in addition to water depth considerations, may then favour selection of a floating vessel instead of a fixed installation.

Wind turbines used for the generation of electricity on a large scale are very large and heavy. A 10MWwind turbine will typically have blades that are each 90m in length and the Towerhead Mass of the wind turbine, including the blades, generator, and gearbox would be in the region of 500 to 670 tonnes. The support structure for the wind turbine, including the tower is accordingly also very large, as is the floating vessel upon which the wind turbine support structure and the wind turbine are mounted. The size of the wind turbine support structure and the size of the floating vessel pose many challenges, including being able to manufacture and install them at a reasonable cost, so that electricity is generated at a cost that is competitive, relative to other methods of generating electricity. The floating vessel and the turbine support structure must meet these cost challenges, whilst still being capable of withstanding the loads exerted by the wind on the wind turbine and the loads exerted by the sea on the floating vessel. Consequently, a structurally efficient arrangement must be employed.

In the offshore oil and gas sector, it was found that crane barges having semi-submersible hulls had significantly lower motions in seas, compared to barges with ship-shaped hulls. In the same way, most floating drilling rigs have semi-submersible hulls, having lower motions in seas, compared to drill ships. In both cases, low motions are desirable. A floating vessel that supports a wind turbine must also have low motions in seas because the wind turbine and its blades cannot accept high motions or angles of inclination. Hence many of the wind turbine vessels that are proposed for offshore use or that are in use offshore have hulls of semi-submersible design. All of the above types of floating vessel are moored during operation.

A key reason for the motions of semi-submersibles being lower relates to their hydrodynamic performance. The hull of any non-submerged floating vessel pierces through the still water level. This area, seen on plan, for example at the surface of a sea, is the cut waterplane area. As a wave passes the floating vessel it causes variations in the distribution of buoyancy of the vessel, and these variations induce vessel motions. These variations are lower if the cut waterplane area is lower, as is the case if the vessel has a semi-submersible hull.

Most types of floating vessel, for example cargo ships and ferries, must move forwards at an acceptable speed. For these types of floating vessel speed becomes a more important design consideration compared to floating vessel motions. Semi-submersible hulls tend to have high drag, meaning that they are not streamlined and cannot move forwards quickly. So vessels that require an acceptable forward speed have ship-shaped hulls.

If the floating vessel is moored it remains desirable that drag on the hull, during the passage of waves, is minimised because high drag leads to high loads in the mooring system and consequent increased cost of the system. Most floating wind vessels in operation today have hulls consisting of vertical cylinders and these have high drag. The second embodiment of the floating vessel of this invention has the joint advantages of having a submersible hull, but with the buoyant members of the floating vessel being ship-shaped in form, so that drag is reduced and loads in the mooring system are reduced, during the passage of waves. This is made possible by providing the second embodiment of the floating vessel with a turret mooring system so that the floating vessel heads into the prevailing seas. On the other hand, it is to costly to design, provide and maintain a turret mooring system. These costs are inapplicable to the first embodiment of the invention and so overall this first embodiment might be selected, depending partially on predicted possible extreme conditions at the offshore site and, to an extent, on the preference of the operator.

Accordingly, the present invention provides in a first aspect a floating structure for supporting a horizontal axis wind turbine, the floating structure having a submersible hull, at least a first buoyant body, a second buoyant body and a third buoyant body attached to the submersible hull and a wind turbine tower support structure, the floating structure is assembled from a first sub-assembly and a second sub-assembly that are each independently buoyant and independently stable when floating on water, the first sub-assembly comprises an elongate submersible first pontoon to which the first buoyant body is attached, the first pontoon that has a longitudinal axis X-X and that forms a first part of the submersible hull and the second subassembly comprises an elongate submersible second pontoon to which the second buoyant body and the third buoyant body are attached, the second pontoon has a longitudinal axis Y-Y and that forms a second part of the submersible hull, the first pontoon is orientated during assembly so that its longitudinal axis X-X is perpendicular to the longitudinal axis Y-Y of the second pontoon and so that a first end of the first pontoon is facing towards a first side wall of the second pontoon, the first pontoon is provided at its first end with a first connecting feature of a connection arrangement which is connected during assembly to a second connecting feature of the connection arrangement that is provided as part of the first side wall of the second pontoon, the first side wall of the second pontoon is located substantially within a single plane that is orientated during assembly to be perpendicular to the axis X-X of the first pontoon, the first pontoon further comprises at least a first ballast tank located on one side of its longitudinal axis X-X and at least a second ballast tank located on the other side of its longitudinal axis X-X so that, during assembly, at least the heel of the first sub-assembly can be adjusted, the second pontoon further comprises at least a third ballast tank located on one side of its longitudinal axis Y-Y and at least a fourth ballast located on the other side of its longitudinal axis Y-Y so that, during assembly, at least the heel of the second sub-assembly can be adjusted.

The present invention according to this first aspect provides advantages over prior art floating structures because the provision of ballast tanks means that the sub-assemblies can be independently buoyant and stable, even when all of the buoyant bodies and the tower support structure are provided on them. This has the result of reducing the amount of assembly that needs to be done on the water. It is only necessary to weld the two sub-assemblies together to assembly the floating structure.

Preferably, the first buoyant body, the second buoyant body, the third buoyant body, the first pontoon and the second pontoon each have a form comprising flat faces such that they can be fabricated from flat plate steel.

It is advantageous to fabricate from flat steel plate because this avoids the need for expensive fabrication machinery, such as is needed to roll the plate for large diameter cylinders.

Preferably, the first sub-assembly is a tower pontoon sub-assembly and the wind turbine tower support structure is attached to the first pontoon towards one end of the first pontoon and the first buoyant body is attached to the first pontoon towards its other end.

Preferably, the connection arrangement is a cantilevered connection, the first pontoon is provided at its first end with at least one cantilever arm and the second pontoon is provided with a connecting arch section in the vicinity of which a keel plate of the second pontoon extends upwardly to an arm mounting plate such that the arm mounting plate is located above the keel plate of the second pontoon and above a keel plate of the first pontoon, wherein, during assembly the at least one cantilever arm is connected to the arm mounting plate.

Preferably, the first pontoon comprises at least one further ballast tank in addition to the first ballast tank and the second ballast tank.

Preferably, the second pontoon comprises in addition to the third ballast tank and the fourth ballast tank a fifth ballast tank located on the same side of the longitudinal axis Y-Y as the third ballast tank and a sixth ballast tank located on the same side of the longitudinal axis Y-Y as the fourth ballast tank.

Preferably, the wind turbine tower support structure comprises a conical transition piece that is connected at its lower end a top deck of the first pontoon and is connected at its upper end to a tower attachment flange.

Preferably, there are two cantilever arms.

Preferably, the first pontoon has an aft bulkhead and the two cantilever arms extend outwardly from the aft bulkhead and each have a mounting pad on their upper surface.

Preferably, the second connecting feature of the connection arrangement that is provided as part of the first side wall of the second pontoon, comprises an opening in the first side wall wherein, the first pontoon and the second pontoon are connected together by infill plates that are welded to each of them and wherein the interior of the first pontoon and the interior of the second pontoon are connected by the opening in the first side wall.

This arrangement is advantageous because the opening in the side wall provides welders and other personnel with space to work inside the pontoons, e.g. during the welding operations of the assembly process.

Preferably, the submersible hull is T-shaped and wherein the first pontoon represents the upright part of the T and the second pontoon represents the cross-piece of the T. Preferably, the second pontoon has a constant beam.

It is advantageous for the second pontoon to have a constant beam because it is easier to fabricate it and because it makes the second sub-assemblies easier to stack together during transportation. The space taken up by the second sub-assemblies is reduced, thus reducing transportation costs.

Preferably, the second pontoon has a starboard spacing section on one side of a connecting arch section and a port spacing section on the other side of the connecting arch section wherein a keel plate is provided on each of the starboard spacing section, the connecting arch section and the port spacing section that is in the same plane as a keel plate provided on the first pontoon and wherein the connecting arch section has a top deck that is at the same level as the top deck of the first pontoon and the starboard spacing section and the port spacing section have a top deck that is lower than the top deck of the pontoon.

Preferably, the second pontoon has a constant beam and a second side wall that is parallel to the first side wall, and wherein the second buoyant body and the third buoyant body are each columnar and have a width that is the same as or smaller than the beam of the second pontoon and each extend upwardly from the second pontoon, wherein the second buoyant body is located at or towards one end of the second pontoon and the third buoyant body is located at or towards the other end of the second pontoon.

Preferably, the first buoyant body is columnar and has a width that is the same as or smaller than the beam of the first pontoon and extends upwardly from the first pontoon.

Preferably, the first buoyant body, the second buoyant and the third buoyant body are columns which have an octagonal cross-sectional profile.

According to a second aspect of the present invention, the first buoyant body, the second buoyant body and the third buoyant body are barges with ship-shaped hulls.

In the second aspect of the present invention the first buoyant body is preferably provided with a turret mooring.

The second aspect of the present invention is suitable for mooring to a geostationary mooring.

The geostationary mooring may be of any suitable type, for example an internal or external turret mooring or a disconnectable turret mooring. In all of the appropriate systems there is a geostationary part, to which the mooring lines and subsea electrical cable or cables are attached. The floating structure freely rotates about the geostationary part. There may be any number of mooring lines or chains, e.g. three, six or nine.

Preferably, the first pontoon has a beam which varies along its length from the largest beam at a turbine support section to a narrowest beam at a buoyant body support section.

Preferably the floating structure further comprises a horizontal axis wind turbine having a turbine tower connected to the wind turbine tower support structure.

According to a third aspect of the present invention there is provided a method of assembling a floating structure wherein in a first stage of assembly the first sub-assembly is floated on water and ballasted to have a first draft and the second sub-assembly is floated on water and ballasted to have a second draft, wherein during the first stage of assembly the first draft and the second draft are selected so that the at least one cantilever arm of the first pontoon can be located under the arm mounting plate of the second pontoon without touching it, and the at least one cantilever arm is located underneath the arm mounting plate, wherein in a second stage of assembly that follows the first stage of assembly the first sub-assembly and the second sub-assembly are provided with drafts which place the at least one cantilever arm into contact with the arm mounting plate, whereby during the second stage of assembly adjacent portions of the first sub-assembly and the second sub-assembly that are above the surface of the water are connected together.

Preferably, during the second stage of assembly the first sub-assembly is de-ballasted so that to its draft decreases.

Preferably, the method of assembling a floating structure further comprises a third stage of assembly, which follows the second stage of assembly, in which the draft of the first subassembly and the second sub-assembly are reduced relative to their drafts in the second stage of assembly, such that adjacent portions of the first sub-assembly and the second sub-assembly that were below the surface of the water during the second stage of assembly are now above the surface of the water and are connected together.

Preferably, during the third stage of assembly the arm mounting plate is located above the surface of the water and at least a portion of the at least one cantilever arm is located above the water and the arm mounting plate and the at least one cantilever arm are connected together.

Preferably, the method of assembling a floating structure further comprises a fourth stage of assembly of attaching the wind turbine to the wind turbine support structure, wherein during the fourth stage of assembly the floating structure is ballasted so that it has sufficient stability for attachment of the wind turbine. The floating structure has sufficient stability to be able to support the wind turbine, e.g. the wind turbine tower, the wind turbine nacelle and the wind turbine blades, whilst the floating structure is afloat with a draft of approximately 6 metres.

This enable the floating structure to be afloat in water depths that are commonly found in ports and in the departure channels from ports.

During the first stage of assembly the drafts of the first and second sub-assemblies are selected so that the two sub-assemblies can be floated, on their own buoyancies, to meet one another. Then one sub-assembly is deballasted so as to make the connection between them, aided by the connection arrangement. The parts of the two sub-assemblies that are above water are then welded to one another, if necessary using infill plates. As a result of the two sub-assemblies now being connected to one another the entire assembly has sufficient stability to enable it to be deballasted to lightship draft. This deballasting is carried out and in the second stage of assembly the lower parts of the connections are above water, to enable welding of these lower parts, above water. In a third assembly stage the entire assembly is ballasted to a deeper draft, to ensure adequate stability for mounting the tower 21 and turbine and blades.

The various aspects of the present invention will be described below, with reference to the following figures: Figure 1 is a perspective view of a floating wind turbine vessel with a wind turbine according to a first embodiment of the present invention; Figure 2 is a perspective view of the submersible hull, buoyant columns and wind turbine support of the floating wind turbine vessel of Figure 1; Figure 3 is a perspective view of the tower pontoon sub-assembly of the vessel of Figure Figure 4 is a plan view of the tower pontoon sub-assembly of the vessel of Figure 1; Figure 5 is a perspective view of the two-column pontoon sub-assembly of the vessel of Figure 1; Figure 6 is an elevation, seen from astern, of the two-column pontoon sub-assembly of the vessel of Figure 1; Figure 7 is a plan view of the two-column pontoon sub-assembly of the vessel of Figure Figure 8 is a longitudinal cross-section illustrating the vessel of Figure 1 during positioning of the tower pontoon sub-assembly relative to the two-column pontoon subassembly (with only the connecting arch section of the two-column pontoon sub-assembly shown); 1; Figure 9 is a perspective view of the vessel of Figure 1 seen from underneath and showing the cantilevered connection; Figure 10 is an elevation of the vessel of Figure 1, seen from astern; Figure 11 is a plan view of the vessel of Figure 1 with the tower pontoon sub-assembly in its final position relative to the two-column pontoon sub-assembly; Figure 12 is a longitudinal cross-section illustrating the vessel of Figure 1 with the tower pontoon sub-assembly in its final position relative to the two-column pontoon subassembly (with only the connecting arch section of the two-column pontoon sub-o assembly shown); Figure 13 is a side elevation illustrating the vessel of Figure 1, with a wind turbine mounted on it, and ballasted to its maximum, operating, draft; Figure 14 is a perspective view of a floating wind turbine vessel with a wind turbine according to a second embodiment of the present invention; Figure 15 is a perspective view of the submersible hull, barges and wind turbine support of the floating wind turbine vessel of Figure 14; Figure 16 is a perspective view of the tower pontoon sub-assembly of the vessel of Figure 14; Figure 17 is a plan view of the tower pontoon sub-assembly of the vessel of Figure 14; Figure 18 is a perspective view of the two-barge pontoon sub-assembly of the vessel of Figure 14; Figure 19 is a longitudinal cross-section illustrating the vessel of Figure 14 during positioning of the tower pontoon sub-assembly relative to the two-barge pontoon sub-assembly (with only the connecting arch section of the two-barge pontoon sub-assembly shown); Figure 20 is a longitudinal cross-section illustrating the vessel of Figure 14 with the tower pontoon sub-assembly in its final position relative to the two-barge pontoon sub-assembly (with only the connecting arch section of the two-barge pontoon sub-assembly shown); and Figure 21 is a side elevation illustrating the vessel of Figure 14, with a wind turbine mounted on it, and ballasted to its maximum, operating, draft.

A first embodiment of a floating wind turbine vessel 1 for supporting a wind turbine 13, is illustrated in Figure 1. In one scenario contemplated here the vessel 1 is installed in a sea, in an offshore wind farm. The vessel 1 comprises a horizontal submersible hull 3 on which are mounted a centreline buoyant column 5, a starboard buoyant column 7, a port buoyant column 9 and a turbine tower support structure 11. The wind turbine 13 is attached to the turbine tower support structure 11 of the vessel 1 but does not form part of the vessel 1. The starboard side io of the vessel 1 is its right-hand side when looking from the wind turbine 13 towards the centreline buoyant column 5. The submersible hull 3 comprises two parts. The first part is a tower pontoon 15 on which the turbine tower support structure 11 and the centreline buoyant column 5 are mounted. The tower pontoon 15, the turbine tower support structure 11 and the centreline buoyant column 5 together make up a tower pontoon sub-assembly 16. The second part of the submersible hull 3 is a two-column pontoon 17 on which the starboard buoyant column 7 and the port buoyant column 9 are mounted. The two-column pontoon 17, the starboard buoyant column 7 and the port buoyant column 9 together make up a two-column pontoon sub-assembly 18. The tower pontoon 15 and the two-column pontoon 17 are connected together such that the tower pontoon 15 extends perpendicularly away from the two-column pontoon 17 from the midpoint of the two-column pontoon 17, i.e. midway between the starboard buoyant column 7 and the port buoyant column 9.

The tower pontoon sub-assembly 16 and the two-column pontoon sub-assembly 18 can be ballasted so that each of them is stable when afloat on its own buoyancy. The vessel 1 is provided with a means (not shown) for adding or removing ballast water, for example a water pump, or, for ballasting, the means can facilitate controlled free flooding.

The centreline buoyant column 5, the starboard buoyant column 7, the port buoyant column 9, the tower pontoon 15 and the two-column pontoon 17 are constructed from stiffened flat steel plate. The centreline buoyant column 5, the starboard buoyant column 7 and the port buoyant column 9 each have an octagonal cross-sectional profile that remains constant throughout their height and they all have the same cross-sectional profile and the same height. The tower pontoon 15 and the two-column pontoon 17 have rectangular transverse cross-sectional profiles.

The tower pontoon 15 has a longitudinal axis X-X and the two-column pontoon 17 has a longitudinal axis Y-Y. The axis X-X and the axis Y-Y are perpendicular to each other. The centreline buoyant column 5, the starboard buoyant column 7 and the port buoyant column 9 each have a longitudinal axis Z-Z that is perpendicular to the axis X-X and the axis Y-Y. The centreline buoyant column 5 is located at the forward end of the tower pontoon 15 and the wind turbine support 11 is located towards the other end of the tower pontoon 15. The starboard buoyant column 7 is located at one end of the two-column pontoon and the port buoyant column 9 is located at the other end. The tower pontoon 15 and the two-column pontoon 17 are connected together using a cantilevered connection 19, for example as shown in Figures 3, 4 and 9. A spread mooring (not shown) is used to moor the vessel 1 to the seabed.

The turbine tower support structure 11 is supported by the tower pontoon 15 and is located along the axis X-X. A turbine tower 21 of the wind turbine 13 is attached to the turbine tower support structure 11 when the wind turbine 13 is mounted on the vessel 1. The turbine tower 21 can be removably attached to, or attached in a permanent manner to, the turbine tower support structure 11. The turbine tower 21 is hollow, tapered inwardly from its base and is equipped internally with a ladder (not shown) and/or a mechanical lift to provide access to the wind turbine 13, for example for maintenance or repair work.

The wind turbine 13 is a centreline, horizontal axis, type with a generator (not shown) which is located within a nacelle 23 and to which are attached three wind turbine blades 25 (the wind turbine 13 could, in a variant, be a downwind, horizontal axis type).

The wind turbine 13 rotates on top of the turbine tower 21, to face into the wind. Thus, none of the vessel's three buoyant columns 5,7,9 can be said to necessarily be centreline.

Furthermore, the vessel 1 does not have a true bow in the normal naval architectural sense but, for convenience, centreline buoyant column 5 is termed the centreline buoyant column because it is on the centreline of the tower pontoon 15 that connects it to the turbine tower support structure 11. The centreline buoyant column 5 is at the bow and the starboard buoyant column 7 and the port buoyant column 9 are astern of it and at either end of the two-column pontoon 17, as illustrated, although other positions for the centreline buoyant column 5, the starboard buoyant column 7 and the port buoyant column 9 are envisaged without departing from the present invention.

The turbine tower 21 and the turbine tower support structure 11 have a combined length that supports a nacelle 23 of the wind turbine 13 so that the bottom of the nacelle 23 is approximately 150m above the level of the surface of the sea. In the same way as virtually all wind turbines operate, the wind turbine blades 25 rotate about a horizontal axis that can be orientated in any direction relative to the position in which the vessel 1 is moored. The horizontal axis is parallel to a top deck 27 of the tower pontoon 15 and to a top deck 29 of the two-column pontoon 17. The blades 25 rotate in a plane that is upwind of the wind turbine tower as shown (or, in a variant, they can be in a plane that is downwind of the wind turbine tower 21 (not shown)).The nacelle 23 can rotate, or yaw, relative to the turbine tower 21 about a vertical axis of rotation, so that the nacelle 23 can be oriented such that its longitudinal axis L-L is aligned with the longitudinal axis X-X of the tower pontoon 15 and so that it points towards the centreline buoyant column 5, or so that its longitudinal axis L-L is at an angle to the longitudinal axis X-X of the tower pontoon 15.

A subsea electrical cable (not shown) is attached to the vessel 1 and runs down to the seabed and then to shore, or to a high voltage transformer platform at the wind turbine farm, or to another vessel, if there is an array of vessels.

The tower pontoon 15, seen in close-up view in Figure 2 when attached to the two-column pontoon 17 and in Figure 3 and Figure 4 separately, is a generally hollow structure which comprises four sections along its length. A turbine support section 31, a transitional section 33, that runs between the turbine support section 31 and a midships region 35, and a centreline buoyant column support section 37 at the other end of the midships region 35. The turbine support section 31 has an aft bulkhead 39 that extends perpendicularly downwardly from the top deck 27 to a keel plate 41 of the tower pontoon 15 and that extends transversely to the longitudinal axis X-X between a starboard side 43 and a port side 45 of the tower pontoon 15. A pair of cantilever arms 47 extend outwardly from each bottom corner of the aft bulkhead 39 in a direction parallel to axis X-X, as will be described in further detail below.

The tower pontoon 15 has a rectangular cross-sectional profile that changes along its length. The beam of the tower pontoon 15 narrows in steps along its length from the aft bulkhead 39, where the beam is greatest, to the centreline buoyant column support section 37, where the beam narrows towards a forward bulkhead 26. The top deck 27 is flat and is parallel to the keel plate 41, which is also flat. The starboard side 43 and the port side 45 of the tower pontoon are vertical and run perpendicularly between the top deck 27 and the keel plate 41. The tower pontoon 15 has an external form that is substantially symmetrical about its longitudinal axis X-X in a transverse direction.

The internal volume of the generally hollow tower pontoon 15 is divided into a number of discrete compartments by watertight bulkheads that run transversely to the longitudinal axis X-X of the tower pontoon 15. Those discrete compartments form ballast tanks into which ballast can be added or removed. For example seawater can be pumped into, or flooded into, the ballast tanks and can be pumped out of them. Ballast can be added to all of the ballast tanks, or to just some of them. An example arrangement of watertight bulkheads can be seen in Figure 3 and Figure 4. A first bulkhead 48 is located at the interface between the turbine support section 31 and the transitional section 33 and a second bulkhead 50 is located at the interface between the midships section 35 and the centreline buoyant column support section 37, as illustrated by the dashed lines in Figure 3 and Figure 4. The tower pontoon 15 is therefore divided into a first ballast tank 52 formed between the aft bulkhead 39 and the first bulkhead 48, into a second ballast tank 54 formed between the first bulkhead 48 and the io second bulkhead 50 and into a third ballast tank 56 formed between the second bulkhead 50 and the forward bulkhead 26.

The first ballast tank 52 is divided in half lengthwise by a dividing bulkhead 58. In a variant, the dividing bulkhead 58 could be located in the second ballast tank 54, or in the third ballast tank 56, or two or more of the ballast tanks 52,54,56 could be provided with a dividing bulkhead 58. The dividing bulkhead 58 divides the first ballast tank 52 into a starboard ballast tank 60 and a port ballast tank 62. A pumping means (not shown) for pumping ballast seawater into and out of the starboard ballast tank 60 and the port ballast tank 62 is provided to facilitate adjustment of the buoyancy of the tower pontoon 15 as a whole and in order to provide the tower pontoon 15 with a different buoyancy on its starboard side to its port side, for example to help make the tower pontoon sub-assembly 16 stable during the assembly process, for example to remove any heel, so that it floats on an even keel.

The turbine support section 31 has a constant beam of 15m and a depth of 10m. The midships section 35 has a constant beam of 12m and a depth of 10m (or in a variant a depth of 8m).

The transitional section 33 has a beam that reduces at a constant rate from 15m at its interface with the turbine support section 31 to 12m at its interface with the midships section 35. The centreline buoyant column support section 37 is provided at its end with a faceted profile having five sides that match, and are aligned with, five of the sides of the octagonally shaped centreline buoyant column 5. The centreline buoyant column 5 is aligned so that one of its eight sides is aligned facing forwards and orientated transversely and perpendicularly to the axis X-X, i.e. that one of the eight sides is aligned with the forward bulkhead 26.

The turbine tower support structure 11 extends upwardly from the top deck 27 of the tower pontoon 15 and it comprises an upwardly tapering conical transition piece 49 which has at its upper end a circular cross-sectional profile tower attachment flange 51. The centre-point of the tower attachment flange 51 is aligned with the axis X-X and is spaced by a distance A of 10m along the axis X-X from the aft bulkhead 39. The diameter of the transition piece 49 at its interface with the top deck 27 is 13m. In a variant of the present invention, the transition piece 49 could be fabricated as an assembly of stiffened flat plates, for example forming an octagon, provided with a cylindrical piece at its top, to receive the circular flange at the base of the tower 21. In a further variant, the transition piece 49 could simply be a cylinder, instead of a conical transition piece 49.

The two-column pontoon 17, seen in close-up view in Figure 2, when attached to the tower pontoon 15, and in Figure 5, Figure 6 and Figure 7 separately, is a generally hollow structure io which comprises five sections along its length. In sequence, from one end of the two-column pontoon 17 to the other, the sections are a starboard buoyant column support section 53, a starboard spacing section 55, a connecting arch section 57, a port spacing section 59 and a port buoyant column support section 61.

The two-column pontoon 17 has a rectangular cross-sectional profile with a beam that remains constant along its length. The top deck 29 is horizontal and flat and rises upwardly in the connecting arch section 57. A keel plate 63 is also horizontal and flat and runs parallel to the top deck 29, so that the keel plate 63 also rises upwardly in the connecting arch section 57. The forward side 65 and the aft side 67 of the two-column pontoon 17 are vertical and run perpendicularly between the top deck 29 and the keel plate 63. The two-column pontoon 17 has an external form that is substantially symmetrical about its longitudinal axis Y-Y in a transverse direction.

The internal volume of the generally hollow two-column pontoon 17 is divided into a number of discrete compartments by watertight bulkheads that run transversely to the longitudinal axis Y-Y of the two-column pontoon 17. Those discrete compartments form ballast tanks into which ballast can be added or removed, for example seawater can be pumped into, or flooded into, the ballast tanks and can be pumped out of them. Ballast can be added to all of the ballast tanks, or to just some of them. An example arrangement of watertight bulkheads can be seen in Figure 5, Figure 6 and Figure 7. A third bulkhead 68 is located at the interface between the starboard buoyant column support section 53 and the starboard spacing section 55, a fourth bulkhead 70 is located at the interface between the starboard spacing section 55 and the connecting arch section 57, a fifth bulkhead 72 is located between the connecting arch section 57 and the port spacing section 59 and a sixth bulkhead 74 is located between the port spacing section 59 and the port buoyant column support section 61, as illustrated by the dashed lines in Figure 5, Figure 6 and Figure 7. The two-column pontoon 17 is therefore divided into a fourth ballast tank 76 formed between the starboard end of the two-column pontoon 17 and the third bulkhead 68, a fifth ballast tank 78 formed between the third bulkhead 68 and the fourth bulkhead 70, a sixth ballast tank 80 formed between the fifth bulkhead 72 and the sixth bulkhead 74 and a seventh ballast tank 82 formed between the sixth bulkhead 74 and the port end of the two-column pontoon 17. In this embodiment there is no ballast tank between the fourth bulkhead 70 and the fifth bulkhead 72 because the forward facing side 65 has an open section where the tower pontoon 15 joins to it.

The fifth ballast tank 78 is divided in half lengthwise by a dividing bulkhead 84. The sixth ballast tank 80 is divided in half lengthwise by a dividing bulkhead 86. In a variant, the dividing io bulkheads 84,86 could be located in the fourth ballast tank 76 and the seventh ballast tank 82 respectively, or three or more of the ballast tanks 76,78,80,82 could be provided with a dividing bulkhead. The dividing bulkhead 84 divides the fifth ballast tank 78 into a forward starboard ballast tank 88 and an aft starboard ballast tank 90. The dividing bulkhead 86 divides the sixth ballast tank 60 in a forward port ballast tank 92 and an aft port ballast tank 94. A pumping means (not shown) for pumping ballast seawater into and out of the forward ballast tanks 88,92 and the aft ballast tanks 90,94 is provided to facilitate adjustment of the buoyancy of the two-column pontoon 17 as a whole and in order to provide the two-column pontoon 17 with a different buoyancy on its forward side to its aft side, for example to help make the two-column sub-assembly 18 stable during the assembly process, for example to remove any heel, so that zo it floats on an even keel.

The two-column pontoon 17 has a constant beam of 12m and a constant depth of 8m. The starboard buoyant column support section 53 and the port buoyant column support section 61 each has a length of 12m. The starboard spacing section 55 and the port spacing section 59 each has a length of 20m. The connecting arch section has a length of 27m. The top deck 29 of the arch connecting section is located 2m above the top deck 29 of the starboard spacing section 55 and the port spacing section 59. The keel plate 63 of the arch connecting section 57 is located 2m above the keel plate 63 of the starboard spacing section 55 and the port spacing section 59. The starboard buoyant column support section 53 and the port buoyant column support section 61 are provided at their ends with a faceted profile having five sides that match, and are aligned with, five of the sides of the octagonally shaped starboard buoyant column 7 and port buoyant column 9 respectively. The starboard buoyant column 7 and port buoyant column 9 are aligned so that one of their eight sides is aligned facing forwards and orientated parallel to the axis Y-Y, i.e. that one of the eight sides is aligned with the forward facing side 65 of the two-column pontoon 17.

The tower pontoon 15 and the two-column pontoon 17 are connected together at the cantilevered connection 19, as can be seen from Figure 8, Figure 9, Figure 10 and Figure 11. Figure 8 is a longitudinal cross-section illustrating the vessel 1 during positioning of the tower pontoon sub-assembly 16 relative to the two-column pontoon sub-assembly 18, with only the connecting arch section 15 of the two-column pontoon sub-assembly 18 shown for the purposes of clarity. The two-column pontoon sub-assembly 18 is shown located adjacent to a quayside. The tower-pontoon sub-assembly 16 is not yet de-ballasted to the final position in which the keel plate 41 of the tower pontoon 15 and the keel plate 63 of the two-column pontoon 17 are in the same horizontal plane. Figure 9 is a perspective view from underneath io the vessel 1 showing the cantilevered connection 19. Figure 10 is an elevation from astern of the vessel 1 showing the cantilevered connection 19. Figure 11 is a plan view of the vessel 1 with the two-column pontoon 17 made transparent in order to see the cantilevered connection 19. Figure 12 is a longitudinal cross-section illustrating the vessel of Figure 1 with the tower pontoon sub-assembly 16 in its final position relative to the two-column pontoon sub-assembly 18, with only the connecting arch section 57 of the two-column pontoon 18 shown for the purposes of clarity.

The cantilevered connection 19 is made by attaching the two cantilever arms 47 provided on the tower pontoon 15 to a horizontal arm mounting plate 69 provided on the connecting arch section 57 of the two-column pontoon 17. The connecting arch section 57 comprises a central horizontal section 71, from either side of which an inclined section 73 slopes downwardly to join up with the starboard spacing section 55 or the port spacing section 59 respectively. The arm mounting plate 69 is provided on the underside of the central horizontal section 71, i.e. on the keel plate 63. The cantilever arms 47 extend perpendicularly from each bottom corner of the aft bulkhead 39 on the tower pontoon 15 and they each have on their uppermost sides and at their free ends a flat and horizontal mounting pad 75 that is parallel to the top deck 27. The cantilever arms 47 have a length of 6.8m, i.e. a length that is just over half the beam of the two-column pontoon 17. During assembly, which will be described in detail below, the horizontal mounting pads 75 of the cantilever arms 47 are brought into contact with the arm mounting plate 69 on the two-column pontoon 17 and the aft bulkhead 39 on the two pontoon is aligned with the forward facing side 65 of the two-column pontoon, with a separation of 0.2m between the aft bulkhead 39 and the forward facing side 65. The mounting pad 75 on each cantilever arm 47 is joined to the connecting arch section 57 with a suitable connection, such as a bolted or welded connection.

Aspects of the floating wind turbine vessel 1 method of manufacture, method of assembly and procedure for towing out to sea for installation in an offshore windfarm will now be described.

The vessel 1 is initially fabricated in two sub-assemblies, a first sub-assembly being the tower pontoon sub-assembly 16 and a second sub-assembly being the two-column pontoon subassembly 18. The tower pontoon sub-assembly 16 and the two-column pontoon sub-assembly 18 can be fabricated in any suitable location, such as a fabrication shop, which may or may not be at a port, or such as a dry dock.

Once the tower pontoon sub-assembly 16 and the two-column pontoon sub-assembly 18 have been fabricated into the forms shown, for example, in Figure 3 and in Figure 5 respectively, io the two-column pontoon sub-assembly 18 is transported to a quayside and then lifted using a crane or cranes and placed into the sea next to the quayside. In a variant, if the two-column pontoon sub-assembly 18 is fabricated in a dry-dock then it can be floated out from the dry dock and towed to a quayside.

The two-column pontoon sub-assembly 18 is ballasted to a draft B of 3.5m, as shown in Figure 6 and in Figure 8, by pumping or flooding seawater into the ballast tanks 76,78,80,82. The two-column pontoon sub-assembly 18 is ballasted such that it floats in a stable manner on the sea, without the need for any external support. If necessary, the heel of the two-column pontoon sub-assembly 18trimcan be adjusted by having different amounts of ballast in the ballast tanks, for example different amounts of ballast in the forward ballast tank 88 and the aft ballast tank 90 of the fifth ballast tank 78 and different amounts of ballast in the forward ballast tank 92 and the aft ballast tank 94 of the sixth ballast tank 80 forward ballast tanks 88,92 and the aft ballast tanks 90,94.

The tower pontoon sub-assembly 16 is also transported to the quayside and lifted using a crane or cranes and placed into the sea next to the quayside in the vicinity of the two-column pontoon sub-assembly 18. In a variant, if the tower pontoon sub-assembly 16 is fabricated in a dry-dock then it can be floated out from the dry dock and towed to a quayside.

The tower pontoon sub-assembly 16 is ballasted to an initial draft C of 4m, as shown in Figure 3 and in Figure 8, by pumping or flooding seawater into the ballast tanks 52,54,56. The tower pontoon sub-assembly 16 is ballasted such that it floats in a stable manner on the sea, without the need for any external support. If necessary, the heel of the tower pontoon sub-assembly 16, and/or its trim, can be adjusted by having different amounts of ballast in the starboard ballast tank 60 and the port ballast tank 62. The initial draft C of the tower pontoon sub-assembly 16 is 0.5m greater than the draft B of the two-column pontoon sub-assembly 18.

The tower pontoon sub-assembly 16 and the two-column pontoon sub-assembly can then be joined together. The tower pontoon sub-assembly 16 is towed into place so that it is spaced apart from and orientated perpendicularly to the two-column pontoon sub-assembly 18 and so that the longitudinal axis X-X of the tower pontoon 15 is midway between the starboard buoyant column 7 and the port buoyant column 9. The tower pontoon sub-assembly 16 is then moved towards the two-column sub-assembly 18 and the cantilever arms 47 of the tower pontoon sub-assembly 16 pass under the connecting arch section 57 of the two-column pontoon 17. The aft bulkhead 39 of the tower pontoon 15 is located at a distance of 0.2m from the forward side 65 of the two-column pontoon 17. The cantilevered connection 19 is then lo made by de-ballasting the tower pontoon sub-assembly 16 so that it rises up in the water to a draft B of 3.5m, i.e. the same draft as the two-column pontoon sub-assembly 18, so that the mounting pad 75 on each of the cantilever arms 47 comes into contact with the arm mounting plate 69 on the keel plate 63 of the connecting arch section 57 of the two column-pontoon 17. Temporary holding features, such as bumpers, guides and wires (not shown), are then attached between the tower pontoon sub-assembly 16 and the two-column pontoon sub-assembly 18 in order to maintain them in the correct position whilst the work of permanently connecting them together takes place.

The first stage in making the permanent connection is to weld 0.2m wide steel infill plates so as to weld together those parts of the tower pontoon 15 and the two-column pontoon 17 that are located above the surface of the water, i.e. the top deck 27, the top deck 29 and portions of the starboard side 43, the port side 45 and the forward facing side 65. Upon completion of the welding together of those parts, the tower pontoon sub-assembly 16 and the two-column pontoon 18 have greater stability, as they now form in combination the vessel 1. Consequently, a second stage in making the permanent connection can take place after the tower pontoon sub-assembly 16 and the two-column pontoon 18 have been de-ballasted so that the vessel 1 has a shallow draft at which the arm mounting plate 69 and the mounting pad 75 of each cantilever arm 47 are located above the water (not shown). The mounting pads 75 can then be welded, or bolted, or both welded and bolted, to the arm mounting plate 69, and the portions of the starboard side 43, the port side 45 and the forward facing side 65 that were located below the surface of the water in the first stage can now be welded together using 0.2m wide steel infill plates.

This two-stage approach makes the welding procedure as straight-forward as possible and avoids the needs for the complicated arrangements that would otherwise be needed to facilitate welding of parts below the surface of the water.

Upon completion of the first stage and the second stage of making the permanent connection the vessel 1 is now ready to have a wind turbine 13 installed on it. In order to increase the stability of the vessel 1 during the installation of the wind turbine 13 seawater ballast can be added into one or more of the ballast tanks 52,54,56,76,78,80,82. The addition of this ballast increases the draft of the vessel 1 to a draft E of 6m. The turbine tower 21 is lifted by a crane on to the turbine tower support structure 11 and it is bolted to the tower attachment flange 51. The crane then lifts the nacelle 23 on to the turbine tower 21, the nacelle 23 is attached to the turbine tower 21 and then the crane lifts each of the three turbine blades 25 for attachment to the nacelle 23. It is necessary to actively manage the ballast of the vessel 1 during installation io of the various components of the wind turbine 13. The mass of each component, when no longer supported by a crane but solely supported by the vessel 1, produces a downwards force on the vessel 1. Thus, ballast must be removed from the tower pontoon sub-assembly 16 and/or the two-column pontoon sub-assembly 18 in order to maintain the desired draft.

The addition of the wind turbine 13 means that the vessel 1 is now ready to be towed to an offshore wind farm. The vessel 1 can be towed at the draft E of 6m. It is envisaged that the vessel 1 can be towed at greater drafts, including up to the maximum draft F of the vessel 1. At the maximum operating draft of the vessel 1 the top surfaces of the buoyant columns (5,7,9) are located above the surface of the water, as shown in Figure 13.

The ability to tow the vessel 1 to the installation site means that there is no need to load the vessel 1 on to a heavy lift ship and to unload it from that heavy lift ship at the installation site. This avoids the high expense of utilising a heavy lift ship.

Before the vessel 1 is towed to its offshore installation site, the spread mooring system (or soft mooring system) (not shown) is pre-laid by installing the seabed anchors or piles and laying the mooring chains or wires (not shown). When the vessel 1 has arrived at the offshore wind farm it is connected to the spread mooring system and connected to a subsea electrical cable to prepare it for the generation and export of electricity. The vessel 1 is ballasted to its maximum, operating, draft F of 20m, as shown in Figure 13. When the vessel 1 is moored the nacelle 23 will yaw and take up the correct heading, i.e. facing directly into the wind.

Ballast is held within at least one of the ballast tanks 52,54,56,76,78,80,82 of the tower pontoon sub-assembly 16 and the two-column pontoon assembly 18 whilst they are floating on, or submerged in, water.

Assembly of the vessel 1 from two sub-assemblies 16,18 has an additional advantage of facilitating fabrication of those sub-assemblies 16,18 at a location remote from a port at which they will be assembled for subsequent towing to the offshore wind farm. Parts of the subassemblies 16,18 can be fabricated inland and transported to the port by road or by rail, for assembly onshore at the port. They may also be fabricated in an overseas country and be transported to the port on a heavy lift ship. The reduced size of the sub-assemblies 16,18 compared to the assembled vessel 1 means that such transportation is feasible.

It is envisaged that in order to support a wind turbine 13 having a 15MW generation capability io the vessel 1 would be sized so that each of the tower pontoon sub-assembly 16 and the two-column pontoon sub-assembly 18 would weigh in the region of 2,000 tonnes.

A second embodiment of a floating wind turbine vessel 201 for supporting a wind turbine 13, is illustrated in Figure 14. In one scenario contemplated here the vessel 201 is installed in a sea, in an offshore wind farm. The vessel 201 comprises a horizontal submersible hull 203 to which are connected a forward barge 205, a starboard outer barge 207, a port outer barge 209 and a turbine tower support structure 211. A turret mooring 212 is attached to the forward barge 205. The wind turbine 13 is attached to the turbine tower support structure 211 of the vessel 201 but does not form part of the vessel 201. The starboard side of the vessel 201 is its right-hand side when looking from the wind turbine 13 towards the forward barge 205. The submersible hull 203 comprises two parts. The first part is a tower pontoon 215 on which the turbine tower support structure 211 and the forward barge 205 are mounted. The tower pontoon 215, the turbine tower support structure 211 and the forward barge 205 together make up a tower pontoon sub-assembly 216. The second part of the submersible hull 203 is a two-barge pontoon 217 to which the starboard outer barge 207 and the port outer barge 209 are attached. The two-barge pontoon 217, the starboard outer barge 207 and the port outer barge 209 together make up a two-barge pontoon sub-assembly 218. The tower pontoon 215 and the two-barge pontoon 217 are connected together such that the tower pontoon 215 extends perpendicularly away from the two-barge pontoon 217 from the midpoint of the two-barge pontoon 217, i.e. between the starboard outer barge 207 and the port outer barge 209.

The tower pontoon sub-assembly 216 and the two-barge pontoon sub-assembly 218 can be ballasted so that each of them is stable when afloat on its own buoyancy. The vessel 201 is provided with a means (not shown) for adding or removing ballast water, for example a water pump, or, for ballasting, the means can facilitate controlled free flooding.

The forward barge 205, the starboard outer barge 207, the port outer barge 209, the tower pontoon 215 and the two-barge pontoon 217 are constructed from stiffened flat steel plate. The forward barge 205, the starboard outer barge 207 and the port outer barge 209 each have a ship-shaped profile that remains constant throughout their height. The tower pontoon 215 has a generally rectangular cross-sectional profile with a tapering bow portion. The two-barge pontoon 17 has a rectangular transverse cross-sectional profile.

The tower pontoon 215 has a longitudinal axis X-X and the two-barge pontoon 17 has a longitudinal axis Y-Y. The axis X-X and the axis Y-Y are perpendicular to each other. The forward barge 205 is located at the forward end of the tower pontoon 215 and the wind turbine support 211 is located towards the other end of the tower pontoon 215. The starboard outer barge 207 is located at the starboard end of the two-barge pontoon 217 and the port outer barge 209 is located at the port end of the two-barge pontoon 217. The tower pontoon 215 and the two-barge pontoon 217 are connected together using a cantilevered connection 219, for example as shown in Figures 16, 17, 19 and 20. A mooring (not shown) is connected to the turret mooring 212 to moor the vessel 201 to the seabed.

The turbine tower support structure 211 is supported by the tower pontoon 215 and is located along the axis X-X. A turbine tower 21 of the wind turbine 13 is attached to the turbine tower support structure 211 when the wind turbine 13 is mounted on the vessel 1. The turbine tower 21 can be removably attached to, or attached in a permanent manner to, the turbine tower support structure 211. The turbine tower 21 is hollow, tapered inwardly from its base and is equipped internally with a ladder (not shown) and/or a mechanical lift to provide access to the wind turbine 13, for example for maintenance or repair work.

The wind turbine 13 is a centreline, horizontal axis, type with a generator (not shown) which is located within a nacelle 23 and to which are attached three wind turbine blades 25 (the wind turbine 13 could, in a variant, be a downwind, horizontal axis type). The wind turbine 13 rotates on top of the turbine tower 21, to face into the wind.

The turbine tower 21 and the turbine tower support structure 211 have a combined length that supports a nacelle 23 of the wind turbine 13 so that the bottom of the nacelle 23 is approximately 150m above the level of the surface of the sea. In the same way as virtually all wind turbines operate, the wind turbine blades 25 rotate about a horizontal axis that can be orientated in any direction relative to the vessel 201. The horizontal axis is parallel to a top deck 227 of the tower pontoon 215 and a top deck 229 of the two-barge pontoon 217. The blades 25 rotate in a plane that is upwind of the wind turbine tower as shown (or, in a variant, they can be in a plane that is downwind of the wind turbine tower 21 (not shown)).The nacelle 23 can rotate, or yaw, relative to the turbine tower 21 about a vertical axis of rotation, so that the nacelle 23 can be oriented such that its longitudinal axis L-L is aligned with the longitudinal axis X-X of the tower pontoon 215 and so that it points towards the forward barge 205, or so that its longitudinal axis L-L is at an angle to the longitudinal axis X-X of the tower pontoon 215.

A subsea electrical cable (not shown) is attached to the vessel 201 and runs down to the seabed and then to shore, or to a high voltage transformer platform at the wind turbine farm, io or to another vessel, if there is an array of vessels.

The tower pontoon 215, seen in close-up view in Figure 15 when attached to the two-barge pontoon 217 and in Figure 16 and Figure 17 separately, is a generally hollow structure which comprises four sections along its length. A turbine support section 231, a transitional section 233, that runs between the turbine support section 231 and a midships region 235, and a bow section 237 at the other end of the midships region 235. The turbine support section 231 has an aft bulkhead 239 that extends perpendicularly downwardly from the top deck 227 to a keel plate 241 of the tower pontoon 215 that extends transversely to the longitudinal axis X-X between a starboard side 243 and a port side 245 of the tower pontoon 215. A pair of cantilever arms 247 extend perpendicularly outwardly from each bottom corner of the aft bulkhead 239, as will be described in further detail below.

The turbine support section 231, the transitional section 233 and the midships section 235 of the tower pontoon 215 each has a rectangular cross-sectional profile. The beam of the tower pontoon 215 narrows in steps along its length from the aft bulkhead 239, where the beam is greatest, to the midships section 235, where the beam is narrowest. The bow section 235 tapers. The top deck 227 is flat and is parallel to the keel plate 241, which is also flat. The starboard side 243 and the port side 245 of the tower pontoon 215 are vertical and run perpendicularly between the top deck 227 and the keel plate 241. The tower pontoon 215 has an external form that is substantially symmetrical about its longitudinal axis X-X in a transverse direction.

The internal volume of the generally hollow tower pontoon 215 is divided into a number of discrete compartments by watertight bulkheads that run transversely to the longitudinal axis X-X of the tower pontoon 215. Those discrete compartments form ballast tanks into which ballast can be added or removed. For example, seawater can be pumped into, or flooded into, the ballast tanks and can be pumped out of them. Ballast can be added to all of the ballast tanks, or to just some of them. An example arrangement of watertight bulkheads can be seen in Figure 16 and Figure 17. A first bulkhead 248 is located at the interface between the turbine support section 231 and the transitional section 233 and a second bulkhead 250 is located at the interface between the midships section 235 and the bow section 237, as illustrated by the dashed lines in Figure 16 and Figure 17. The tower pontoon 215 is therefore divided into a first ballast tank 252 formed between the aft bulkhead 239 and the first bulkhead 248, into a second ballast tank 254 formed between the first bulkhead 248 and the second bulkhead 250 and into a third ballast tank 256 formed between the second bulkhead 250 and the front of the bow section 237.

The first ballast tank 252 is divided in half lengthwise by a dividing bulkhead 258. In a variant, the dividing bulkhead 258 could be located in the second ballast tank 254, or in the third ballast tank 256, or two or more of the ballast tanks 252,254,256 could be provided with a dividing bulkhead 258. The dividing bulkhead 258 divides the first ballast tank 252 into a starboard ballast tank 260 and a port ballast tank 262. A pumping means (not shown) for pumping ballast seawater into and out of the starboard ballast tank 260 and the port ballast tank 262 is provided to facilitate adjustment of the buoyancy of the tower pontoon 215 as a whole and in order to provide the tower pontoon 215 with a different buoyancy on its starboard side to its port side, for example to help make the tower pontoon sub-assembly 216 stable during the assembly process, for example to remove any heel, so that it floats on an even keel.

The turbine support section 231 has a constant beam of 15m and a depth of 10m. The midships section 235 has a constant beam of 12m and a depth of 10m (or in a variant a depth of 8m). The transitional section 233 has a beam that reduces at a constant rate from 15m at its interface with the turbine support section 231 to 12m at its interface with the midships section 235.

The turbine tower support structure 211 extends upwardly from the top deck 227 of the tower pontoon 215 and it comprises an upwardly tapering conical transition piece 249 which has at its upper end a circular cross-sectional profile tower attachment flange 251. The centre-point of the tower attachment flange 251 is aligned with the axis X-X and is spaced by a distance A of 10m along the axis X-X from the aft bulkhead 239. The diameter of the transition piece 249 at its interface with the top deck 227 is 13m.

The two-barge pontoon 217, seen in close-up view in Figure 15 when attached to the tower pontoon 215 and in Figure 18 separately, is a generally hollow structure which comprises three sections along its length. In sequence, from one end of the two-barge pontoon 217 to the other, the sections are a starboard spacing section 255, a connecting arch section 257 and a port spacing section 259.

The two-barge pontoon 217 has a rectangular cross-sectional profile with a beam that remains constant along its length. The top deck 229 is horizontal and flat and rises upwardly in the connecting arch section 257. A keel plate 263 is also horizontal and flat and runs parallel to the top deck 29, so that the keel plate 263 also rises upwardly in the connecting arch section 257. The forward side 265 and the aft side 267 of the two-barge pontoon 217 are vertical and run perpendicularly between the top deck 229 and the keel plate 263. The two-barge pontoon 217 has an external form that is substantially symmetrical about its longitudinal axis Y-Y in a transverse direction.

The internal volume of the generally hollow two-barge pontoon 217 is divided into a number of discrete compartments by watertight bulkheads that run transversely to the longitudinal axis Y-Y of the two-barge pontoon 217. Those discrete compartments form ballast tanks into which ballast can be added or removed, for example seawater can be pumped into, or flooded into, the ballast tanks and can be pumped out of them. Ballast can be added to all of the ballast tanks, or to just some of them. An example arrangement of watertight bulkheads can be seen in Figure 18. A third bulkhead 268 is located at the starboard end of the two-barge pontoon 217, a fourth bulkhead 270 is located at the interface between the starboard spacing section 255 and the connecting arch section 257, a fifth bulkhead 272 is located between the connecting arch section 257 and the port spacing section 259 and a sixth bulkhead 274 is located between the port spacing section 259 and the port end of the two-barge pontoon 217, as illustrated by the dashed lines in Figure 18. The two-barge pontoon 217 is therefore divided into a fourth ballast tank 276 formed between the third bulkhead 268 and the fourth bulkhead 270 and a fifth ballast tank 278 formed between the fifth bulkhead 272 and the sixth bulkhead 274. In this embodiment there is no ballast tank between the fourth bulkhead 270 and the fifth bulkhead 272 because the forwards facing side 265 has an open section where the tower pontoon 215 joins to it.

The fourth ballast tank 276 is divided in half lengthwise by a dividing bulkhead 284. The fifth ballast tank 278 is divided in half lengthwise by a dividing bulkhead 286. The dividing bulkhead 284 divides the fourth ballast tank 276 into a forward starboard ballast tank 288 and an aft starboard ballast tank 290. The dividing bulkhead 86 divides the fifth ballast tank 278 into a forward port ballast tank 292 and an aft port ballast tank 294. A pumping means (not shown) for pumping ballast seawater into and out of the forward ballast tanks 288,292 and the aft ballast tanks 290,294 is provided to facilitate adjustment of the buoyancy of the two-barge pontoon 217 as a whole and in order to provide the two-barge pontoon 217 with a different buoyancy on its forward side to its aft side, for example to help make the two-barge subassembly 218 stable during the assembly process, for example to remove any heel, so that it floats on an even keel.

The two-barge pontoon 217 has a constant beam of 12m and a constant depth of 8m. The starboard spacing section 255 and the port spacing section 259 each has a length of 20m. The connecting arch section 257 has a length of 27m. The top deck 229 of the arch connecting section 257 is located 2m above the top deck 229 of the starboard spacing section 255 and io the port spacing section 259. The keel plate 263 of the arch connecting section 257 is located 2m above the keel plate 263 of the starboard spacing section 255 and the port spacing section 259.

The tower pontoon 215 and the two-barge pontoon 217 are connected together at the cantilevered connection 219, as can be seen from Figures 19 and 20. Figure 19 is a longitudinal cross-section illustrating the vessel 201 during positioning of the tower pontoon sub-assembly 216 relative to the two-barge pontoon sub-assembly 218, with only the connecting arch section 257 of the two-barge pontoon sub-assembly 218 shown for the purposes of clarity. The two-barge pontoon sub-assembly 218 is shown located adjacent to a quayside. The tower-pontoon sub-assembly 216 is not yet de-ballasted to the final position in which the keel plate 241 of the tower pontoon 215 and the keel plate 263 of the two-barge pontoon 217 are in the same horizontal plane.. Figure 20 is a longitudinal cross-section illustrating vessel 201 with the tower pontoon sub-assembly 216 in its final position relative to the two-barge pontoon sub-assembly 218, with only the connecting arch section 257 of the two-barge pontoon 218 shown for the purposes of clarity.

The cantilevered connection 219 is made by attaching the two cantilever arms 247 provided on the tower pontoon 215 to a horizontal arm mounting plate 269 provided on the connecting arch section 257 of the two-barge pontoon 217. The connecting arch section 257 comprises a central horizontal section 271, from either side of which an inclined section 273 slopes downwardly to join up with the starboard spacing section 255 or the port spacing section 259 respectively. The arm mounting plate 269 is provided on the underside of the central horizontal section 71, i.e. on the keel plate 263. The cantilever arms 247 extend perpendicularly from each bottom corner of the aft bulkhead 239 on the tower pontoon 215 and they each have on their uppermost sides and at their free ends a flat and horizontal mounting pad 275 that is parallel to the top deck 27. The cantilever arms 247 have a length of 6.8m, i.e. a length that is just over half the beam of the two-barge pontoon 217. During assembly, which will be described in detail below, the horizontal mounting pads 275 of the cantilever arms 247 are brought into contact with the arm mounting plate 269 on the two-barge pontoon 217 and the aft bulkhead 239 on the two pontoon is aligned with the forward facing side 265 of the two-barge pontoon, with a separation of 0.2m between the aft bulkhead 239 and the forward facing side 265. The mounting pad 275 on each cantilever arm 247 is joined to the connecting arch section 257 with a suitable connection, such as a bolted or welded connection.

Aspects of the floating wind turbine vessel 201 method of manufacture, method of assembly io and procedure for towing out to sea for installation in an offshore windfarm will now be described.

The vessel 201 is initially fabricated in two sub-assemblies, a first sub-assembly being the tower pontoon sub-assembly 216 and a second sub-assembly being the two-barge pontoon sub-assembly 218. The tower pontoon sub-assembly 216 and the two-barge pontoon sub-assembly 218 can be fabricated in any suitable location, such as a fabrication shop, which may or may not be at a port, or such as a dry dock.

Once the tower pontoon sub-assembly 216 and the two-barge pontoon sub-assembly 218 have been fabricated into the forms shown, for example in Figure 16 and in Figure 18 respectively, the two-barge pontoon sub-assembly 218 is transported to a quayside and then lifted using a crane or cranes and placed into the sea next to the quayside. In a variant, if the two-barge pontoon sub-assembly 218 is fabricated in a dry-dock then it can be floated out from the dry dock and towed to a quayside.

The two-barge pontoon sub-assembly 218 is ballasted to a draft B of 3.5m, as shown in Figure 19, by pumping or flooding seawater into the ballast tanks 276,278. The two-barge pontoon sub-assembly 218 is ballasted such that it floats in a stable manner on the sea, without the need for any external support. If necessary, the heel of the two-barge pontoon sub-assembly 218, and/or its trim, can be adjusted by having different amounts of ballast in the ballast tanks, for example different amounts of ballast in the forward ballast tank 288 and the aft ballast tank 290 of the fourth ballast tank 276 and different amounts of ballast in the forward ballast tank 292 and the aft ballast tank 294 of the fifth ballast tank 278.

The tower pontoon sub-assembly 216 is also transported to the quayside and lifted using a crane or cranes and placed into the sea next to the quayside in the vicinity of the two-barge pontoon sub-assembly 218. In a variant, if the tower pontoon sub-assembly 216 is fabricated in a dry-dock then it can be floated out from the dry dock and towed to a quayside.

The tower pontoon sub-assembly 216 is ballasted to an initial draft C of 4m, as shown in Figure 19, by pumping or flooding seawater into the ballast tanks 252,254,256. The tower pontoon sub-assembly 216 is ballasted such that it floats in a stable manner on the sea, without the need for any external support. If necessary, the heel of the tower pontoon sub-assembly 216, and/or its trim, can be adjusted by having different amounts of ballast in the starboard ballast tank 260 and the port ballast tank 262. The initial draft C of the tower pontoon sub-assembly 16 is 0.5m greater than the draft B of the two-barge pontoon sub-assembly 218.

The tower pontoon sub-assembly 216 and the two-barge pontoon sub-assembly 218 can then be joined together. The tower pontoon sub-assembly 216 is towed into place so that it is spaced apart from and orientated perpendicularly to the two-barge pontoon sub-assembly 218 and so that the longitudinal axis X-X of the tower pontoon 215 is midway between the starboard outer barge 207 and the port buoyant outer barge 209. The tower pontoon subassembly 216 is then moved towards the two-barge sub-assembly 218 and the cantilever arms 247 of the tower pontoon sub-assembly 216 pass under the connecting arch section 257 of the two-barge pontoon 217. The aft bulkhead 239 of the tower pontoon 215 is located at a distance of 0.2m from the forward side 265 of the two-barge pontoon 217. The cantilevered connection 219 is then made by de-ballasting the tower pontoon sub-assembly 216 so that it rises up in the water to a draft B of 3.5m, i.e. the same draft as the two-barge pontoon subassembly 218, so that the mounting pad 275 on each of the cantilever arms 247 comes into contact with the arm mounting plate 269 on the keel plate 263 of the connecting arch section 257 of the two barge-pontoon 217. Temporary holding features, such as bumpers, guides and wires (not shown), are then attached between the tower pontoon sub-assembly 216 and the two-barge pontoon sub-assembly 218 in order to maintain them in the correct position whilst the work of permanently connecting them together takes place.

The first stage in making the permanent connection is to weld 0.2m wide steel infill plates so as to weld together those parts of the tower pontoon 215 and the two-barge pontoon 2217 that are located above the surface of the water, i.e. the top deck 227, the top deck 229 and portions of the starboard side 243, the port side 245 and the forward facing side 265. Upon completion of the welding together of those parts, the tower pontoon sub-assembly 216 and the two-barge pontoon 218 have greater stability, as they now form in combination the vessel 201.

Consequently, a second stage in making the permanent connection can take place after the tower pontoon sub-assembly 216 and the two-barge pontoon 218 have been de-ballasted so that the vessel 201 has a shallow draft at which the arm mounting plate 269 and the mounting pad 275 of each cantilever arm 247 are located above the water (not shown). The mounting pads 75 can then be welded, or bolted, or both welded and bolted, to the arm mounting plate 269, and the portions of the starboard side 243, the port side 245 and the forward facing side 265 that were located below the surface of the water in the first stage can now be welded together using 0.2m wide steel infill plates.

This two-stage approach makes the welding procedure as straight-forward as possible and avoids the needs for the complicated arrangements that would otherwise be needed to to facilitate welding of parts below the surface of the water.

Upon completion of the first stage and the second stage of making the permanent connection the vessel 201 is now ready to have a wind turbine 13 installed on it. In order to increase the stability of the vessel 201 during the installation of the wind turbine 13 seawater ballast can be added into one or more of the ballast tanks 252,254,256,276,278. The addition of this ballast increases the draft of the vessel 201 to a draft E of 6m. The turbine tower 21 is lifted by a crane on to the turbine tower support structure 211 and it is bolted to the tower attachment flange 251. The crane then lifts the nacelle 23 on to the turbine tower 21, the nacelle 23 is attached to the turbine tower 21 and then the crane lifts each of the three turbine blades 25 for attachment to the nacelle 23. It is necessary to actively manage the ballast of the vessel 201 during installation of the various components of the wind turbine 13. The mass of each component, when no longer supported by a crane but solely supported by the vessel 201 produces a downwards force on the vessel 201. Thus, ballast must be removed from the tower pontoon sub-assembly 216 and/or the two-barge pontoon sub-assembly 218 in order to maintain the desired draft.

The addition of the wind turbine 13 means that the vessel 201 is now ready to be towed to an offshore wind farm. The vessel 1 can be towed at the draft X of 6m. It is envisaged that the vessel 201 can be towed at greater drafts, including up to the maximum draft F of the vessel 201.

The ability to tow the vessel 201 to the installation site means that there is no need to load the vessel 201 on to a heavy lift ship and to unload it from that heavy lift ship at the installation site. This avoids the high expense of utilising a heavy lift ship.

Before the vessel 201 is towed to its offshore installation site, a mooring system is pre-laid by installing the seabed anchors or piles and laying the mooring chains or wires (not shown).

When the vessel 201 has arrived at the offshore wind farm it is connected to the mooring system using the mooring turret 212 and connected to a subsea electrical cable to prepare it for the generation and export of electricity. The vessel 201 is ballasted to its maximum, operating, draft F of 20m, as shown in Figure 21. When the vessel 201 is moored the nacelle 23 will yaw and take up the correct heading, i.e. facing directly into the wind.

Ballast is held within at least one of the ballast tanks 52,54,56,76,78 of the tower pontoon subassembly 216 and the two-barge pontoon assembly 218 whilst they are floating on, or submerged in, water.

Assembly of the vessel 201 from two sub-assemblies 216,218 has an additional advantage of facilitating fabrication of those sub-assemblies 216,218 at a location remote from a port at which they will be assembled for subsequent towing to the offshore wind farm. Parts of the sub-assemblies 216,218 can be fabricated inland and transported to the port by road or by rail, for assembly onshore at the port. They may also be fabricated in an overseas country and be transported to the port on a heavy lift ship. The reduced size of the sub-assemblies 216,218 compared to the assembled vessel 201 means that such transportation is feasible.

It is envisaged that in order to support a wind turbine 13 having a 15MW generation capability the vessel 201 would be sized so that each of the tower pontoon sub-assembly 216 and the two-barge pontoon sub-assembly 218 would weigh in the region of 2,000 tonnes.

Claims

CLAIMS1. A floating structure (1,201) for supporting a horizontal axis wind turbine (13), the floating structure (1,201) having a submersible hull (3,203), at least a first buoyant body (5,205), a second buoyant body (7,207) and a third buoyant body (9,209) attached to the submersible hull (3,203) and a wind turbine tower support structure (11,211), the floating structure (1,201) is assembled from a first sub-assembly (16,216) and a second sub-assembly (18,218) that are each independently buoyant and independently stable when floating on water, the first sub-assembly (16,216) lo comprises an elongate submersible first pontoon (15,215) to which the first buoyant body (5,205) is attached, the first pontoon (15,215) that has a longitudinal axis X-X and that forms a first part of the submersible hull (3,203) and the second sub-assembly (18,218) comprises an elongate submersible second pontoon (17,217) to which the second buoyant body (7,207) and the third buoyant body (9,209) are attached, the second pontoon (17,217) has a longitudinal axis Y-Y and that forms a second part of the submersible hull (3,203), the first pontoon (15,215) is orientated during assembly so that its longitudinal axis X-X is perpendicular to the longitudinal axis Y-Y of the second pontoon (17,217) and so that a first end (39,239) of the first pontoon (15,215) is facing towards a first side wall (65,265) of the second pontoon (17,217), the first pontoon (15,215) is provided at its first end (39,239) with a first connecting feature of a connection arrangement (19,219) which is connected during assembly to a second connecting feature of the connection arrangement (19,219) that is provided as part of the first side wall (65,265) of the second pontoon (17,217), the first side wall (65,265) of the second pontoon (17,217) is located substantially within a single plane that is orientated during assembly to be perpendicular to the axis X-X of the first pontoon (15,215), the first pontoon (15,215) further comprises at least a first ballast tank (60,260) located on one side of its longitudinal axis X-X and at least a second ballast tank (62,262) located on the other side of its longitudinal axis X-X so that, during assembly, at least the heel of the first sub-assembly (16,216) can be adjusted, the second pontoon (17,217) further comprises at least a third ballast tank (88,288) located on one side of its longitudinal axis Y-Y and at least a fourth ballast tank (90,290) located on the other side of its longitudinal axis Y-Y so that, during assembly, at least the heel of the second sub-assembly (18,218) can be adjusted.
2. A floating structure (1,201) as claimed in claim 1, wherein the first buoyant body (5,205), the second buoyant body (7,207), the third buoyant body (9,209), the first pontoon (15,215) and the second pontoon (17,217) each have a form comprising flat faces such that they can be fabricated from flat plate steel.
3. A floating structure (1,201) as claimed in claim 1 or claim 2, wherein the first sub-assembly (16,216) is a tower pontoon sub-assembly (16,216) and the wind turbine tower support structure (11,211) is attached to the first pontoon (15,215) towards one end of the first pontoon (15,215) and the first buoyant body (5,205) is attached to the first pontoon (15,215) towards its other end.
4. A floating structure (1,201) as claimed in any one of claim 1, claim 2 or claim 3, wherein the connection arrangement (19,219) is a cantilevered connection, the first pontoon (15,215) is provided at its first end (39.239) with at least one cantilever arm (47,247) and the second pontoon (17,217) is provided with a connecting arch section (57,257) in the vicinity of which a keel plate (63,263) of the second pontoon (17,217) extends upwardly to an arm mounting plate (69,269) such that the arm mounting plate (69,269) is located above the keel plate (63,263) of the second pontoon (17,217) and above a keel plate (41,241) of the first pontoon (15,215), wherein, during assembly the at least one cantilever arm (47,247) is connected to the arm mounting plate (69,269).
5. A floating structure (1,201) as claimed in any one of the preceding claims, wherein the first pontoon (15,215) comprises at least one further ballast tank (52,54,56) in addition to the first ballast tank (60,260) and the second ballast tank (62,262).
6. A floating structure (1,201) as claimed in any one of the preceding claims, wherein the second pontoon (17,217) comprises in addition to the third ballast tank (88,288) and the fourth ballast tank (90,290) a fifth ballast tank (92,292) located on the same side of the longitudinal axis Y-Y as the third ballast tank (88,288) and a sixth ballast tank (94,294) located on the same side of the longitudinal axis Y-Y as the fourth ballast tank (90,290).
7. A floating structure (1,201) as claimed in any one of the preceding claims, wherein the wind turbine tower support structure (11,211) comprises a conical transition piece (49,249) that is connected at its lower end a top deck (27,227) of the first pontoon and is connected at its upper end to a tower attachment flange (51,251).
8. A floating structure (1,201) as claimed in any one of claims 4 to 7, wherein there are two cantilever arms (47,247).
9. A floating structure (1,201) as claimed in claim 8, wherein the first pontoon (15,215) has an aft bulkhead (39,239) and the two cantilever arms (47,247) extend outwardly from the aft bulkhead and each have a mounting pad (75) on their upper surface.
10. A floating structure (1,201) as claimed in any one of the preceding claims, wherein the second connecting feature of the connection arrangement (19,219) that is provided as part of the first side wall (65,265) of the second pontoon (17,217), comprises an opening in the first side wall (65,265) wherein, the first pontoon (15,215) and the second pontoon (17,217) are connected together by infill plates that are welded to each of them and wherein the interior of the first pontoon (15,215) and the interior of the second pontoon (17,217) are connected by the opening in the first side wall (65,265)
11. A floating structure (1,201) as claimed in any one of the preceding claims, wherein the submersible hull (3,203) is T-shaped and wherein the first pontoon (15,215) represents the upright part of the T and the second pontoon (17,217) represents the cross-piece of the T.
12. A floating structure (1,201) as claimed in any one of the preceding claims, wherein the second pontoon (17,217) has a constant beam.
13. A floating structure (1,201) as claimed in any one of the preceding claims, wherein the second pontoon (15,215) has a starboard spacing section (55,255) on one side of a connecting arch section (57,257) and a port spacing section (59,259) on the other side of the connecting arch section (57,257) wherein a keel plate (63,263) is provided on each of the starboard spacing section (55,255), the connecting arch section (57,257) and the port spacing section (59.259) that is in the same plane as a keel plate (41,241) provided on the first pontoon (15,215) and wherein the connecting arch section (57,257) has a top deck (29,229) that is at the same level as the top deck (27,227) of the first pontoon (15,215) and the starboard spacing section (55,255) and the port spacing section (59,259) have a top deck (29,229) that is lower than the top deck (27,227) of the pontoon (15,215).
14. A floating structure (1) as claimed in any one of the preceding claims, wherein the second pontoon (17) has a constant beam and a second side wall (67) that is parallel to the first side wall (65), and wherein the second buoyant body (9) and the third buoyant body (9) are each columnar and have a width that is the same as or smaller than the beam of the second pontoon (17) and each extend upwardly from the second pontoon (17), wherein the second buoyant body (7) is located at or towards one end of the second pontoon (17) and the third buoyant body (9) is located at or towards the other end of the second pontoon (17).
15. A floating structure (1) as claimed in any one of the preceding claims, wherein the first buoyant body (5) is columnar and has a width that is the same as or smaller than the beam of the first pontoon (15) and extends upwardly from the first pontoon (15).
16. A floating structure (1) as claimed in any one of the preceding claims, wherein the first buoyant body (5), the second buoyant body (7) and the third buoyant body (9) are columns which have an octagonal cross-sectional profile.
17. A floating structure (201) as claimed in any one of claims 1 to 13, wherein the first buoyant body (205), the second buoyant body (207) and the third buoyant body (209) are barges with ship-shaped hulls.
18. A floating structure (201) as claimed in claim 17, wherein the first buoyant body (205) is provided with a turret mooring (212).
19. A floating structure (1,201) as claimed in any one of the preceding claims, wherein the first pontoon (15,215) has a beam which varies along its length from the largest beam at a turbine support section (31,231) to a narrowest beam at a buoyant body support section.
20. A floating structure (1,201) as claimed in any one of the preceding claims, further comprising a horizontal axis wind turbine (13) having a turbine tower (21) connected to the wind turbine tower support structure (11, 211).
21. A method of assembling a floating structure (1,201) according to any one of the claims 4 to 20 wherein in a first stage of assembly the first sub-assembly (16,216) is floated on water and ballasted to have a first draft and the second sub-assembly (18,218) is floated on water and ballasted to have a second draft, wherein during the first stage of assembly the first draft and the second draft are selected so that the at least one cantilever arm (47,247) of the first pontoon (15,215) can be located under the arm mounting plate (69,269) of the second pontoon (17,217) without touching it, and the at least one cantilever arm (47,247) is located underneath the arm mounting plate (69,269), wherein in a second stage of assembly that follows the first stage of assembly the first sub-assembly (16,216) and the second sub-assembly (18,218) are provided with drafts which place the at least one cantilever arm (47,247) into contact with the arm mounting plate (69,269), whereby during the second stage of assembly adjacent portions of the first sub-assembly (16,216) and the second sub-assembly (18,218) that are above the surface of the water are connected together.
22. A method of assembling a floating structure (1,201) as claimed in claim 21, wherein during the second stage of assembly the first sub-assembly (16,216) is de-ballasted so that its draft decreases.
23. A method of assembling a floating structure (1,201) as claimed in claim 21 or claim 22, further comprising a third stage of assembly, which follows the second stage of assembly, in which the draft of the first sub-assembly (16,216) and the second sub-assembly (18,218) are reduced relative to their drafts in the second stage of assembly, such that adjacent portions of the first sub-assembly (16,216) and the second subassembly (18,218) that were below the surface of the water during the second stage of assembly are now above the surface of the water and are connected together.
24. A method of assembling a floating structure (1,201) as claimed in claim 23, wherein during the third stage of assembly the arm mounting plate (69,269) is located above the surface of the water and at least a portion of the at least one cantilever arm (47,247) is located above the water and the arm mounting plate and the at least one cantilever arm (47,247) are connected together.
25. A method of assembling a floating structure (1,201) as claimed in any one of claims 21 to 24, further comprising a fourth stage of assembly of attaching the wind turbine (13) to the wind turbine support structure (11,211), wherein during the fourth stage of assembly the floating structure (1,201) is ballasted so that it has sufficient stability for attachment of the wind turbine (13).