The invention is directed to a process to place a wind turbine tower or wind turbine in a vertical position on the sea bed.

Such a process is described in Dutch patent application NL 2018377 of the same applicant. This publication describes a process wherein a wind turbine connected to a submersible framework is submerged to the sea bed. The framework fixes itself to the sea bed and by means of buoyancy and a moveable arm of the framework the mast of the wind turbine is moved from a substantially horizontal position to a vertical position. The vertical wind turbine is subsequently fixed to the sea bed and the framework is returned to the water surface to receive a next wind turbine. An advantage of such a process is that a fully assembled wind turbine can be installed on the sea bed. This avoids having to assemble the generator, hub and blades at sea, which involves the use of stationary vessels provided with heavy duty cranes. A disadvantage however of this process is that the framework requires to submerge to the sea bed and return back to the water surface for every wind turbine to be installed.

US2012/0308358 describes a partially submersible wind turbine transport vessel suited to transport and install floating wind turbines. Such a vessel can transport a wind turbine in a substantially horizontal position from shore to the off-shore location where the wind turbine is installed. At that off shore location the buoyancy of the transport vessel is altered such that the vessel rotates, the stern submerges and the combined vessel-wind turbine arrives in a vertical position. The floating wind turbine is disconnected from the vessel and anchored at its destined off shore location. The buoyancy of the vessel is increased by allowing pressurized air into the water-filled buoyancy tanks resulting in that the vessel returns to its horizontal floating position. The vessel may subsequently be used to fetch a next wind turbine.

DE10332382 described a vessel suited to place the upper end of a wind turbine on a basis which is fixed to the sea bed. The vessel is provided with a substantially horizontal upper end of the wind turbine consisting of part of the upper end of the mast, generator, hub and blades. This upper end is erected using cables running over a beam extending away from the lower end of the mast. This creates a lever which results in that the mast moves from its horizontal position to a vertical position where it connects with the basis. A problem with this method is that the force required to erect the wind turbine is excessive. The size of off shore wind turbines being installed increase and it is questionable if this method would be able to work with these new wind turbines. Further the method requires a very stable vessel when placing the top part on the basis. This is solved by providing a relatively large ship which can partly submerge using ballast tanks.

A problem with the method of DE10332382 is that it requires that the positioning on the basis requires a stable and large vessel and that excessive forces on the cables are required for erecting the top part of the wind turbine onto such a basis.

The present invention provides an improved process which does not have this problem. Process to place a wind turbine tower in a vertical position on a sea bed comprising the following steps,

(a) providing a floating delivery vessel with the wind turbine tower positioned in a substantially horizontal position on the delivery vessel at sea level thereby defining a lower end of the vessel at which the lower end of the wind turbine tower is positioned and an upper end of the vessel at which the upper end of the wind turbine tower is positioned and wherein the delivery vessel comprises of a support for the wind turbine tower, which support has an upper end and lower end corresponding to the upper end and lower end of the wind turbine tower, a submersible part rotatably connected via a substantially horizontal axis to the lower end of the support and a separate floating part which floating part is slidable connected to the support,

(b) submerging towards the sea bed and fixing to the sea bed at least the submersible part such that the upper end of the wind turbine tower is above the sea level and the lower end of the wind turbine tower points towards the sea bed,

(c) erecting the combined support and wind turbine tower to a substantially vertical position along the submerged and substantially horizontal axis at the submersible part by reducing the distance between the submersible part and the floating part resulting in a force which enhances the erecting of the wind turbine tower , and (d) fixing the wind turbine tower to the sea bed.

An advantage of this process is that a wind turbine tower can be positioned vertically on the sea bed in a relatively simple manner. The forces required to erect the wind turbine tower are significantly less than in the referred to prior art process because the weight of the tower is partly compensated by the fact that a large part of the wind turbine tower is submerged. The process is advantageous because the buoyancy forces enhancing the erection of the wind turbine tower are exercised on the wind turbine at the sea level. This results in the largest possible moment, i.e. the distance between the floating compartment and the positioned horizontal axis of rotation at the sea bed. This in contrast with the earlier described embodiments where a possible buoyancy box is fixed at a relative short distance to the positioned horizontal axis of rotation. This results in that the dimensions of such a vessel may be smaller than for example the earlier described pre-installed framework of NL2018377. Smaller dimensions results in that less subsea force, for example by currents, on the submerged parts of the vessel are exercised. This is advantageous because less counter measures will then be required. Counter measures are typically a dynamic positioning system and because of the reduced subsea forces a smaller dynamic positioning system may be used. A further advantage is that no large actuators are required which have to operate below sea level as in the earlier described pre-installed framework of

NL2018377.

A wind turbine tower in the context of the present invention comprises a foundation and a mast and optionally also a generator, hub and blades. The term generator is in this context also meant to refer to a generator housing. Further, the blades will be connected to the rotation axis of the generator by means of a hub. The foundation may comprise anchoring means at its lower end which fix the wind turbine to the sea bed or connecting means to connect the foundation to a basis already present in the sea bed.

In the process of placing a wind turbine tower in a vertical position on the sea bed the delivery vessel suitably moves under and angle with the sea bed towards the sea bed by submersing the lower end of the delivery vessel resulting in that the delivery vessel positions itself under and angle with the sea bed. The submersing of the lower end of the vessel may be achieved by lowering the buoyancy of the submersible part. It may also be achieved by guiding the submersible part along earlier installed jack-up legs or spud poles of the delivery vessel. In step (b) the upper end of the wind turbine tower, which may also comprise the generator and blades, suitably remain above sea level. The speed of submerging and the angle under which the lower end of the delivery vessel submerges can for example be controlled by varying the buoyancy along the length of the delivery vessel. When the submersible part freely moves to the sea bed, ie not along earlier installed jack up legs or spud poles, it may be advantageous to make use of one or more directable thrusters which provide the lower end of the vessel with at least 3 to even 6 degrees of freedom.

While the lower end of the delivery vessel submerges to the sea bed the wind turbine tower itself may move towards the upper end of the delivery vessel along the length of the delivery vessel. This movement may be effected by the buoyancy of the lower end of the wind turbine itself. Such a movement is advantageous in a situation wherein the wind turbine is positioned towards the lower end of the vessel for balancing reasons when the vessel is used to transport the wind turbine at sea level. When such a positioning is too far towards the lower end of the vessel the lower end of the wind turbine tower may be end up too low when the vessel rests on the sea bed. By simultaneously moving the wind turbine tower along the length of the vessel as described while submerging the position of the wind turbine tower is improved.

In step (c) the erecting of the combined support and wind turbine tower is enhanced by moving the floating part towards the submersible part. The floating part may be pulled below sea level and a resultant force on the connected support will result in that the combined support and wind turbine tower move to a vertical position. The lower end of the support will rotate along the substantially horizontal axis. This axis preferably is suitably close to the sea bed as possible allowing an unobstructed movement of the lower end of the wind turbine relative to the sea bed. Higher elevated axis of rotation are of course possible but will result in that more power will be required to erect the wind turbine tower. The axis is preferably positioned in one position, meaning that in step (c) the axis of rotation itself does not move in any translational directions. This distance reducing may be achieved by means of cables connecting the submersible part with the floating part. When the submersible part is fixed to the sea bed the wind turbine will be positioned under an angle with the sea bed wherein the upper end of the wind turbine tower, as supported by the floating part, is above sea level. When this distance is shortened the floating part is pulled downwards. Because of the resulting buoyancy forces the floating part will move towards a point above the fixed submersible part and thereby erecting the wind turbine to a vertical position. The substantially horizontal axis is present on the submersible part.

Preferably a further buoyancy box is slidable connected to the support between the floating part and the submersible part. The distance between the floating part and the buoyancy box is now shortened in step (c) by shortening a first cable connecting the floating part with the buoyancy box. The buoyancy box is connected to the submersible part by means of a second cable.

Suitably in step (c) the erecting of the wind turbine tower is enhanced by shortening a cable connected to the upper end (A) of the support and running via an end (C) of a rigid construction to a point (B) at the submersible part. This rigid construction is connected to the submersible part and the end (C) over which the cable runs is positioned at a distance from the horizontal axis of rotation. This positioning results in that (A)(B)(C) form the three corners of a triangle in step (c).

Shortening of this cable be enhanced when this cable is connected to the second cable which runs from the buoyancy box and the submersible part as a joint cable. The buoyancy forces will pull this cable towards the buoyancy box and as a result erect the support and wind turbine tower via the resultant forces exercised on the cables connected to upper end (A) via end (C). This results in that the buoyancy box not only directly pushes the support from below upwards but also pulls the support upwards by the aforementioned cables.

This cable may be further reduced in length when this joint cable runs via two spaced apart blocks of a pully arrangement. By moving the two blocks away from each other the length is reduced by a factor which relates to the number of times the cable runs between these blocks. The forces required to move these blocks apart may be provided by actuators, for example hydraulic or electric-mechanical actuators. Such a pully arrangement is advantageous because smaller buoyancy compartments may then be used, enabling a better control of the forces excreted on the support and/or the process may be used in shallower waters.

Alternatively a rotatable rigid construction is rotatably connected to the submersible part at the horizontal axis of rotation. The rigid structure has an end (C) which end is connected to the upper end (A) of the support by means of a, preferably pre-tensioned, cable or rigid structure having a fixed length. End (C) is also connected to an end of second cable which runs via a point (B) at the submersible part to buoyancy box. End (C) is positioned at a distance from the horizontal axis of rotation such that (A)(B)(C) form the three corners of a triangle in step (c). In step (c) the end (C) of the rigid construction rotates around the horizontal axis of rotation by reducing the length of the second cable.

The second cable may run from the buoyancy box to the end (C) of the rigid structure via two spaced apart blocks of a pully as earlier described above. In step (c) the length of the second cable will then be shortened by moving the two blocks away from each other by means of one or more actuators as described above.

When such a rigid structure is used a hydraulic cylinder may be positioned between an intermediate position on the rotating rigid construction and the submersible part. In step (c) it may occur that the support and wind turbine tower erect due to its buoyancy only. In such a situation it may be preferred to store any pressurised hydraulic fluid obtained in this hydraulic cylinder in a buffer vessel. This stored and pressurised hydraulic fluid may be used at another time to drive the one or more hydraulic cylinders to enlarge the distance between the two blocks of the pully arrangement when cable is shortened. This other time will be when an additional force is required to erect the wind turbine tower in step (c).

The substantially vertical positioned wind turbine tower as obtained in step (c) is suitably lowered onto the sea bed or onto a submerged pre-installed basis in the sea bed in step (d). Fixing the wind turbine to the sea bed may be achieved with anchoring means which are part of the wind turbine. For example as described in the afore mentioned NL2018377, page 5, line 6 - page 5, line 14 and Figures 13-15 and their description.

Examples of suction anchors are described in W013152757. The anchoring means may also be piles or the mast itself, in case a mono-pile is used, which is drilled into the sea bed. An example of a suitable mast having built-in drilling means is described in W02017203023. In case the mast is a framework construction the anchors are suitably present at its three or four corners at their bottom end. Preferably the mast is a tubular mast and the foundation is a so-called mono-pile foundation or a tripod foundation. The tubular mast can have a constant diameter or can have a variable diameter that increases in the direction of the foundation.

When a pre-installed basis is used the wind turbine tower may be provided with a tubular mast. Such a wind turbine tower may be lowered onto a submerged pre-installed basis in the sea bed and connected to the pre-installed basis by means of, for example, a slip joint connection. The anchored base element may be a tubular element which has been anchored into the sea floor using a hydrohammer, preferably a submerged hydrohammer or for example by means of a built-in drilling means is described in W02017203023. Examples of such a pre-installed basis and connector is for example described in US2012137622 and in EP2910686. The pre-installed basis may also be a framework. The connector of such a pre installed basis may be above sea level or at any position between the sea level and the sea bed. In case the connector is positioned below sea level it is preferably positioned as near to the sea bed as possible. By as near as possible is at least meant that the connection between foundation and pre-installed basis is less than 20% away from the sea bed relative to the local average sea depth. The wind turbine may also be provided with a support structure having multiple means to engage with multiple pre-installed anchors and wherein the wind turbine is lowered onto multiple anchors as the submerged pre-installed basis in the sea bed. Such a pre-installed anchor may be installed using a mould to accurately position the anchors relative to each other such that they can engage with the support structure of the wind turbine. The means to engage with multiple pre-installed anchors may also be the earlier referred to slip joint connections.

The invention is also directed to a delivery vessel comprising a support for a wind turbine tower, which support has an upper end and lower end corresponding to the upper end and lower end of the supported wind turbine tower, a submersible part rotatably connected along a substantially horizontal axis to the lower end of the support and a separate floating part which floating part is slidable connected to the support, means suited to reduce the distance between the floating part and the submersible part and wherein the vessel is provided with fixing means allowing the lower end of the vessel to directly or indirectly fix to the sea bed. Such a vessel is preferably used in the process according to the invention.

The delivery vessel may also be used as a crane, wherein the submersible part is fixed to the sea bed and the support is used as the arm, also referred to as boom, of the crane. Because of the buoyancy forces such a crane will have a high lifting capacity as will be further described using Figures 17 and 18.

The delivery vessel and the process according to this invention will be discussed using the following non-limiting Figures 1-18.

Figure 1 shows a floating delivery vessel (1) according to the invention provided with a support (la) for a wind turbine tower (3). The wind turbine tower (3) is provided with a generator (3e) and blades (3f) and is positioned substantially horizontal. This enables an easy transport of the wind turbine tower (3) at sea level (15) to the position where the tower is to be installed on the sea bed (16) . The support (la) has an upper end (lc) and lower end (Id) corresponding to the upper end (3c) and lower end (3d) of the supported wind turbine tower (3). A submersible part (5) is rotatably connected along a substantially horizontal axis (7) to the lower end (Id) of the support (la). A separate floating part (12) and a buoyancy box (6) are both slidable connected to the support (la). Floating part (12), buoyancy box (6) and submersible part (5) all float and provide the required buoyancy for the entire vessel (1). Floating part (12) is connected by a cable (8a) to buoyancy box (6). A winch (12b) is present which can reduce the distance between floating part (12) and buoyancy box (6) and thus in effect also reduce the distance between floating part (12) and submersible part (5). Wheels (12a) allow floating part (12) to slide along the lower end of the support (la) and wheels (6a) allow buoyancy box (6) to slide along the lower end of the support (la). The cable (8) connects end (C) of a rotatable rigid beam (2b) via hinges B and O to buoyancy box (6). A is connected via a fixed length pre-tensioned cable (10) to the upper end (A) of the support (la). Further elements shown in Figure 1 will be described when discussing Figures 2-8. Rigid beam (2b) is rotatable connected to axis (7) at its other end.

Figure 2 shows the delivery vessel (1) in a position where the submersible part (5) is submerged towards the sea bed and fixed to the sea bed by anchors (5a) as the fixing means. The upper end (3c) of the wind turbine tower(3) is above the sea level (15) and the lower end of the wind turbine tower (3) points towards the sea bed (16). The cable (8a) is fully pulled in resulting in that floating part (12) and buoyancy box (6) form one buoyancy element. End (C) which is positioned at a distance from the horizontal axis (7) and wherein a third cable (10) runs from the upper end (A) of the support (la) to end (C). The, preferably pre-tensioned, cable (10) with tension S10 (see figure 3) between the upper end (A) of the support (la) and the upper end (C) of the rigid beam (2b) together with the rigid beam (2b) and the support (la) form a rigid triangle ACO. The connection between the buoyancy box (6) and the end (C) of rigid beam (2b) is realized in using cables (8c) and (8). Cable (8) runs from buoyancy box (6) via pully arrangement (13a) at rotation point (O) to pully (9b) at (B) to end (C). The buoyancy force B12 and B6 on both floating compartment (12) and buoyancy box (6) respectively will pull at cable (8) at a force B12*sina and B6*sina (see Figure 3). This combined force will pull at end (C) resulting in a rotation of the triangle AOC along axis (7). This force is increased by increasing the distance between the two blocks (23,24) of pully arrangement (13a) as further explained in Figure 5a and 5b. The support (la) is further erected by the force B6*cosa and B12*cosa exerting a perpendicular force on the support (la) (see also figure 3).

Figure 3 shows forces working on the delivery vessel (1) which realize the rotating moment around rotating point (O). Due to the buoyancy forces B6 of the buoyancy box (6) and B12 of floating part (12) support (la) is erected by the perpendicular force components B6*cosa and B12*cosa and the cable force components B6*sina and B12*sina at end (C) of the rigid beam (2b).

Figure 4 shows the amplifier of the buoyancy force component B*sina of buoyancy box (6) which equalizes the tension in cable (8c). By means of two blocks (19,20) the tension force S8 in cable (8c) will be amplified with a factor il, thus realizing a tension force with magnitude il*S8. Therefore block 19 is coupled to hinge (21) and block (20) is coupled to a rolling hinge (22). Cable (8c) runs over pullies (19a) of block (19) and pullies (20a) of block (20).

Figures 5a and 5b show the pully arrangement (13a) in two positions of block (24) which have been shifted over a distance 613. Cable (8) runs over the pullies (9,9a) of blocks (23,24). Blocks (23,24) are connected to hinges (7) an the rolling hinges (7a). The

transmission factor i2 stands for the extra cable length 613*i2 of cable (8) which will be accumulated in the pully arrangement (13a) when the hydraulic cylinders (13) will shift over a distance 613. The total hydraulic force Fhl3 is equal to il*i2*S8, where il*S8 is the amplified force 58 in cable (8) by a factor il (see figure 4).

Figures 6 and 7 show a hydraulic cylinder (14) which is positioned between an intermediate position on the rotating rigid beam (2b). The hydraulic fluid in cylinder (14) is fluidly connected to a buffer vessel (17). Buffer vessel (17) is drawn out of scale and position for clarity reasons. It may be positioned on the deck of the submersible part (5). Buffer vessel (17) is also fluidly connected to hydraulic cylinders (13) of pully arrangement (13a). This allows storage of pressurised fluid in buffer vessel (17) originating from hydraulic cylinder (14) when the wind turbine tower (3) and support erect solely as a result of the buoyancy forces B6 and B12. When additional force is required the stored pressurised fluid may be used in hydraulic actuators (13) to drive blocks (23,24) apart thereby enhancing the erecting in step (c) by shortening cable length CBOE of cable (8).

In Figure 8 the wind turbine mast (3) and support (la) are fully erected to a vertical position. By using cable (11) which runs via block (11a) at the upper end (lc) of the support (la) it is possible to slowly lower the tower (3) onto, for example, a pre-installed foundation. In this vertical position cable (11) may be connected to cable (8) enabling lowering of the wind turbine tower (3) in a controlled fashion using pully arrangement (13a). In this way the weight of the wind turbine mast (3) may be partly compensated by the buoyancy forces of floating part (12) and buoyancy box (6). A cylinder (not shown) may be present to avoid that the support (la) and wind turbine (3) do not tip over. Figure 9 shows a floating delivery vessel and wind turbine mast as in Figure 1. A difference is that cable (8) runs from the buoyancy box (6) via a points (O) on the

submersible part (5) at the axis of rotation (7), via point B to a point (C) on a rigid structure (2a) to the upper end (A) of the support (la). In contrast with Figure 1 the rigid structure (2a) and end (C) itself will not rotate. The rigid structure (2a) is a framework connected to the deck of the submersible part (5). The cable (8) runs more than one time via two spaced apart blocks (19,20) of a pully arrangement (13a). This arrangement has means, preferably hydraulic means (13), to alter the distance between the two blocks (19,20) and as a consequence alter the effective length OBCA of the joint cable (8). Apart from the buoyancy forces of the buoyancy box (6) and floating part (12) also a winch (10) via cable (11) is connected to the buoyancy box (6), from which a further rotation moment around O can be realized. This is especially advantageous in more shallow waters. A further advantage is that the support (la) will be less loaded as compared to the design shown in Figures 1-8.

Figure 10 shows the buoyancy forces B6 of buoyancy box (6) and B12 of the floating part (12) as well as the winch forces Sll in cable (11) which all contribute to the erecting moment around rotation point (O). The perpendicular forces B6*cosa, B12*cosa and Sll*sin b directly realize the erecting moment around O and the summation of buoyancy forces B12* sina and B6*sina minus the winch component Sll*cos create the resultant tension in cable (8) from which the erecting moment around O can be determined. The pully arrangement (13a) including the hydraulic cylinders (13) and the winch (10), when the submersible part (5) is submerged and anchored on the sea bottom (16), are positioned above sea level (15). This is advantageous because no special measures will then be required to make the equipment suited for submerged use. Cable (8) is optional in this arrangement. In effect cable (11) can suffice as also shown in Figure 16.

In analogy with figure 8 in figure 11 the wind turbine mast (3) and support (la) in figure 11 are fully erected to a vertical position. The difference with figure 8 is that counter buoyancy boxes (6,12) are directly coupled at the upper end (lc) by means of cable (8) via the block (8a) to the fixed hoisting eye (3a) of the wind turbine (3). This enables one to lower the wind turbine tower (3) by relaxing the pully arrangement (13a). Figure 12a shows a vessel having 4 spud poles (35) suited to guide the submersible part from floating to a submerged position on the seabed (16). After erection of the wind turbine and lifting of the submersible part (5) up until sea level (15) from the sea bottom (16) the winch (38) will pull the spud poles out of the sea bottom. All equipment, from which the jack up systems (35,37,38), pumps (33), winches (10), pully arrangement (13a), are advantageously positioned above sea level while the submersible part (5) is located at and anchored to the sea bottom (16).

Figures 12b shows the vessel of Figure 12a from aside when it floats on the sea level. This will be the position when delivery vessel and wind turbine (3) can be transported. Figure 12c shows the vessel of Figure 12a from above. An opening is shown between two trusses as structure (2a) allowing the wind turbine to be positioned on the sea bed when it erects as shown in Figure 12a.

Figures 12d and 12e are similar to figures 12a, 12b and 12c apart from the horizontal rotation line (7) which is shifted to the back side of the submersible part (5). The vessel has a more compact configuration and hinge (B) is not available anymore. Apart from pump (33), which is positioned under water, the rest of the equipment is positioned above sea level (15).

Figures 12d and 12e are similar to figures 12d and 12e, apart from the jack-up boxes (31) which are positioned under sea level (15).

Figure 13a-e show another delivery vessel. This vessel may be positioned using four tugs (40) next to a pre-installed basis (41) for a wind turbine as shown in Figure 13a. Once positioned the spud poles (42) are lowered to anchor the vessel as shown in Figure 13b. By submerging submersible part (45) and shortening the distance between floating part (46) the rotating support (47) erects as shown in Figures 13c and 13d. When the wind turbine (47) is positioned above basis (41) it can be lowered and fixed to said basis (41) as shown in Figure 13e. Figure 13c shows that floating part (46) is connected to the submersible part (45) by means of hoisting cables (54) which run via blocks (55,56) in a pully arrangement. Figure 14 shows an alternative hoisting arrangement for the floating delivery vessel and wind turbine mast. The upper end of support (la) is connected with the upper end of rigid structure (2a) by means of hoisting cables (49) which run via blocks (50, 51) in a pully arrangement. The ends of cable (49) are connected to the top end of the wind turbine and to a winch (53) on the rigid structure (2a) respectively. The floating part (46) is connected to the submersible part (45) by means of hoisting cables (54) which run via blocks (55,56) in a pully arrangement as also shown in Figures 13c and figure 15. Block (55) is connected to the floating part (46) and block (56) is connected to the submersible part (45). The ends of cable (54) is connected to floating part (46) and to a winch (57) via a point (58) on the submersible part (45).

The hoisting arrangement of Figure 14 is also shown in Figure 15. This figure shows the situation wherein the wind turbine (3) is fully erected to a vertical position and wherein the hoisting arrangement is used to lower the wind turbine (3) onto the sea bed and fix to the sea bed. For this cables (49) are extended using winch (53). In order to reduce the power required to operate winch (53) it is preferred to couple winch (53) with winch (57). This may be done by positioning the two winches in line and provide a coupling-decoupling arrangement. In this coupled system the floating part (46) is connected by means of pulley blocks (55, 56) via cables (54) to winch (57), which is connected by means of pulley blocks (50, 50a) to the wind turbine (3) and connected foundation. The floating part (46) will have an upward buoyancy force via cables (54) working on winch (57) and the wind turbine (3) including foundation will have a downward gravity force via cables (49) working on winch (53). Because winch (57) and winch (53) are coupled the net power required to lower the wind turbine (3) is significantly reduced. Thus a less powerful winch may be applied as compared to a situation wherein such coupling is not used. The floating part (46) may be provided with means to let in water and/or air to trim the upward forces exercised upon said floating part (46).

Figure 16 shows an alternative arrangement wherein the floating part (46) is moved along the support (la) by means of a hoisting cable (59) connected to the floating part (46) and at a position (60) on the rigid construction (2a) which is above sea level. This hoisting cable may be arranged as cable (49) in Figure 14. The advantage of this arrangement is that less power is required to erect the wind turbine in the final phase of erecting the turbine. A further advantage is that the pully arrangement for hoisting cable (59) can remain substantially above sea level as shown in this Figure thereby avoiding having to take special measures to make them suitable for submerged operation. In addition to cables (59) a hoisting cable (49) as in Figure 14 is also present.

Figure 17 shows how the delivery vessel (1) can also be used as a heavy load lifting crane (62). For this the support (la) rotates further from its vertical position along axis (7) such that floating part (46) is positioned above axis (7) and free from support (la) as shown. Floating part (46) may have to be disconnected from any sliding arrangement connecting this part to support (la). In this way the support (la) becomes a boom (63) of a crane (62). At the side where a wind turbine (3) was supported is now room to lift heavy loads (61). By connecting winches (53) and (57) as explained above the net forces required for the winch (53) to lift heavy load (61) will be significantly reduced because of the buoyancy forces exercised on cable (54) by floating part (46). Such a heavy load lifting crane is further advantageous because it will have an improved stability because of the lower centre of gravity of the crane relative to the sea bed as compared to cranes positioned on jack-up vessels. Furthermore this crane will exercise a significantly lower vertical load on the supports (42a) and/or spud poles (42) because of the buoyancy exercised by the submerged floating part (46) as compared to the state of the art jack-up vessel. The floating part (46) may be provided with means to let in water and/or air to trim the upward forces exercised upon said floating part (46). The crane (62) as shown can be easily adapted with for example a moving platform to provide a yaw rotational movement of boom (63) relative to the fixed submersible part (45).

Figure 18 shows the relevant hoisting cables, winches and floating part (46) of Figure 17 required to achieve the advantageous crane (62) as described above.

The invention is also directed to the following crane (62) as shown in Figures 17 and

18.

Crane (62) comprising

a submersible part (45), a separate floating part (46),

a boom (63) having an upper end (48) and lower end (47),

wherein the submersible part (45) is rotatably connected along a substantially horizontal axis (7) to the lower end (47) of the boom (63),

wherein means (54) are present suited to reduce the distance between the floating part (46) and the submersible part (45) and

wherein the vessel is provided with fixing means (5a) allowing the submersible part (45) of the crane to be fixed to the sea bed (16).

A rigid (49) construction may be connected to the submersible part (45) and extends vertically upwards from the submersible part (45). Such a structure is advantageous to provide a better arm for hoisting and allows winches and the like to be positioned above sea level.

Suitable a hoisting cable (49) runs from one end suited to attach a load (61) to another end via the upper end (48) of boom (63) and via blocks (50, 51) in a pully arrangement and wherein the other end of cable (49) is connected to a winch (53). The floating part (46) is connected to the submersible part (45) by means of hoisting cables (54) which run via blocks (55,56) in a pully arrangement, wherein block (55) is connected to the floating part (46) and block (56) is connected to the submersible part (45) and wherein one end of cable (54) is connected to a winch (57) and wherein the winch (53) and winch (57) may be coupled to each other and decoupled from each other. Floating part (46) will be present above block (56), positioned at axis (7). The pully arrangement will then in use pull cable (54) upwardly thereby relaxing the coupled winches (53,57) resulting in that winch (57) requires less power to hoist load (61). In a way upwardly moving floating part (46) partly pulls load (61) upward. If the load (61) is not very heavy the upward moving part (46) may fully pull load (61) upwards. The force on cable (54) may be adjusted by varying the buoyancy of floating part (46). The floating part may therefore be provided with means to fill or empty the floating part with water such to influence its buoyancy.

In use the submersible part (45) is fixed to the sea bed (16), floating part (46) is submerged and the upper end (48) of boom (63) is above sea level. When not in use the boom (63) may rotate to a position above the floating part (46). By moving floating part (46) to the end of the boom (63) and by disconnecting the submersible part (45) and increasing its buoyancy a floating crane (63) may be obtained like in Figure 14 which can be

transported by means of for example tugs to a next location.