WO2011102738A2

WO2011102738A2 - A method and vessel for offshore transport and installation of windmill assemblies

Info

Publication number: WO2011102738A2
Application number: PCT/NO2011/000062
Authority: WO
Inventors: Knut Vasstrand
Original assignee: Aker Marine Contractors As
Priority date: 2010-02-18
Filing date: 2011-02-18
Publication date: 2011-08-25
Also published as: NO20100323A1; WO2011102738A3; NO331703B1

Abstract

A method of and a vessel (9) for relocating from an onshore assembly yard a number of complete windmill assemblies (37), each consisting of a tower (22) with generator fixed to a foundation substructure (34), to an offshore site and subsequent installation by a vessel (9) having a split bow (10). The windmill assemblies (37) are skidded over the bow (10) onto the deck of the vessel while standing in an upright position and remaining in that position during transit to the offshore installation site. There each windmill assembly (37) is skidded to the split bow (10) and lowered through the split bow to the seabed by installation means (1-5, 11, 39-45) on the bow while the vessel (9) is kept on station at least partly by a dynamic positioning system. The windmill assemblies (37) are guided and secured as necessary at upper and lower parts thereof in all operating scenarios, including skidding, transport and lowering, and any subsequent recovery for de-commissioning of an earlier installed windmill assembly by reversing the operating procedures above.

Description

A method and vessel for offshore transport and installation of windmill as semblies

TECHNICAL FIELD

The present invention relates to windmills located offshore, and in particular to a method and vessel for transporting and installing such windmills at an offshore location, as defined in the preambles of the independent claims.

BACKGROUND ART

EU's goal to have 20.000 windmills installed offshore within year 2020 is revealing an enormous gap in available installation capacity. For seabed fixed windmills most of the existing concepts require assembly-work to be carried out at the offshore site by crane vessels. For this installation method the most challenging part is to mount piecewise the tower, the very sensitive generator and the delicate blades due to the general weather sensitivity for this method making it very inefficient and costly. The ideal installation vessel should have ample stability and be capable of installing complete windmill assembly and transport as many units as possible per trip with each assembly consisting of foundation structure, tower and turbine with blades where the foundation may be of different type of substructures, typical jacket, mono- or multi-pile, mono-tower or gravity structures of steel or concrete material or anchored floating mono-tower type of substructure of steel or concrete material. From WO 03066427 Al a method and apparatus is known for transporting and installation of erect structures in shallow waters (6 - 20m) by suspending one erect structure at each end of the vessel and by lowering the vessel to seabed by ballasting the hull and actually resting it on the seabed during picking up and / or installation of the structures. This method and apparatus will limit the installation to shal- low water and to install only two structures in each trip to offshore, as well as the need to develop a complete new and novel vessel design.

For seabed fixed windmills most of the existing concepts require assembly work to be carried out at the offshore site by crane vessels. For this installation method the most challenging part is to mount piecewise the tower, the very sensitive generator and the delicate blades due to the general weather sensitivity for this method making it very inefficient and costly. DISCLOSURE OF THE INVENTION

This invention, as defined in the independent claims, eliminates the need to assemble the windmill on the seabed at the wharf for the vessel to pick it up. The invention provides a method and a vessel that, by avoiding ballasting of the vessel down to be resting on seabed during picking up and / or installation of structures, gives the advantage of being able to install a plurality of windmill assemblies per offshore trip and to operate in deeper waters, typically 20 - 60 m for structures standing on seabed and even in deep waters for floating structures. Various types of wide body vessels typical of barge type may be utilised or constructed, or alternatively con- verting existing and suitable commercial vessels typical like tankers, bulk vessels and others that would possibly eliminate the disadvantages of the prior art, in particular the need to develop a specialised vessel and to ballast the vessel to rest on seabed during the operations.

Converting existing vessels and equipping these with dynamical positioning or mooring systems, elevated support guides for sea fastening of the windmills during transport and introducing an installation system on the bow, are enabling the vessel to keep bow or stern against the weather direction, with the ability to maintain the bow in a stable position above the installation point on the seabed with weather up to at least and typical 30 degrees on the bow or stem, in order to lower the windmill assembly by the installation system and land it accurately and safely onto the intended location on seabed. Two such types of commercial typical vessels suitable for conversion are Suezmax and Aframax type of hulls, the latter having a DWT of about 80000 - 120000 T, but any wide-body barge or vessel may be used for conversion, thereby avoiding the need to develop a novel vessel design. By wid- ening the breadth of an Aframax vessel by introducing side pontoons, the hull of this vessel has a new total breadth of about 52 m, giving sufficient stability to survive a summer storm. After conversion, the vessel has the ability to skid onboard over the bow, transport and install typically six windmill-assemblies per trip instead of two and in about 20 - 60 meter of water depth instead of 6 - 20m, with a total height of about 150 m and a weight of about 1500 T for each of the structures.

During transport, the windmill assemblies, each consisting typically of a four-leg jacket structure, a tower and a generator with blades, are supported by support structures on the deck of the vessel. At the offshore installation site, the vessel- bow being made like a split-bow with an opening, is positioned above a set of e.g. four accurately pre-installed piles with their "stick-ups" above the seabed. The an- chor legs of the jacket substructure are fitted with integrated hydraulic actuated internal pile grippers, making it possible to lock the anchor legs and the windmill assembly to skid beams via skid shoes on the anchor legs during transport to the offshore site, and after having been skidded to the split bow, making it possible to lock the jacket legs to the stick-ups on seabed after the legs having been lowered down through the split bow to seabed by one of several alternative installation systems.

One typical installation system is a minimum of four strand bundle jacks, each typically of 600 T capacity, located above the jacket on jacking-decks suspended in two installation towers, one on each side of the split bow. Strand bundle reels are accommodated above the jacking-decks in the installation-towers, and compensators of typically 600 T capacities may be introduced in the lift-bundles between the jacks and the jacket to alleviate dynamic loads during lowering and landing of the windmill assembly. A second alternative system consists of four or more sets of winch and tension compensators with accumulators and with running wire sheaves, all located on the deck of said vessel, and with the lift wire brought to a lift point on top of the windmill substructure via a suspended sheave in top of installation tower, so that the tension in the wire and the vertical movement of the assembly are kept within de- fined limits by said tension compensator.

A third alternative system consists of four or more power cylinders with accumulators vertically supported on the deck of the vessel, with the cylinder rods connected to yokes guided by the installation towers and with four or more fixed and parallel wire lines, with one end anchored on deck and the other end at top of the jacket, run over suspended sheaves on the yokes, thereby transforming the pull or push from the power cylinders to downward or upward motion on the jacket and windmill assembly, and by connecting accumulators to the hydraulic power system, allowing it to work as a compensator so that the tension in the wire and the vertical movement of the assembly are kept within defined limits by said compensator. Other means for compensating or impact damping may be introduced on the landing of the jacket legs on the pile stick-ups to keep the tension in the lowering- bundles or lift wires and the vertical movement and impact of the mill-assembly within defined limitations and under proper control. A guide pin on two sides of the jacket is giving lateral support to the lower part of mill-assembly during lowering by following a guide channel on the side of the installation tower and the split-bow. At the same time, the upper part being the windmill tower will be supported by an articulated truss work with releasable hoop iron from the top of the installation tower. For completing the lowering operations when the guide-pin is out of contact with the guide-channel at the last part of the descent, two or more guideline tensioners are with their guidelines connected to guideline fixtures on top of the pile stick-ups and used for lateral stabilising of the lower part of the windmill assembly in the last part of the descent and for fine alignment of the jacket's anchor legs onto the stick- ups on the piles or on a suction anchored foundation frame. The split-bow or stern is by the dynamic positioning (DP) system on the vessel kept against the wind / waves, with split bow positioned in optimum position for landing the legs of the jacket substructure on stick-ups on the four pre-installed piles, or as an alternative onto the four similar stick-ups located on a pre-installed suction-anchored founda- tion frame.

As an alternative to rotate the vessel itself beyond the said typical 30 degrees on the weather direction, tagger winches are able to rotate the windmill assembly 10- 15 degrees laterally in addition as a last step of alignment of the jacket legs prior to landing onto the stick-ups, depending on the vessel orientation of the jacket legs in relation to the stick-ups. These 15 degrees in addition to the said typical 30 degrees on the weather-direction maintainability, is allowing the vessel to operate most effectively in two of the four quadrants and somewhat less effectively in the other two quadrants, depending on the direction of sea and wind on bow or stern.

The stick-ups on the pre-installed piles or on the suction-anchored foundation frame provide full stability when the windmill assembly and jacket are landed and locked to the stick-ups by the internal pile grippers in the jacket-legs. The articu^¬ lated truss-work with hoop iron is then released from the connection to the windmill tower, and bundles / lift wires and guidelines are disconnected and the vessel bow pulled out from the installation position. The connection of the jacket legs to the stick-ups may then be grouted as a permanent connection. BRIEF DESCRIPTION OF DRAWINGS

The present invention is described in more detail below in the form of non-limiting exemplifying embodiments with reference to the appended drawings, of which Figure 1 is an elevation of a vessel according to the invention;

Figure 2 is part of the elevation in Figure 1 at a larger scale;

Figure 3 is a front view of the vessel of Figure 1 illustrating a first embodiment of the invention;

Figure 4 is a partial front view of the vessel during lowering of a windmill; Figure 5 is a front view with a windmill installed;

Figure 6 is part of the front view of Figure 5 at a larger scale; Figure 7 is a front view illustrating a second embodiment of the invention; and Figure 8 is a front view illustrating a third embodiment of the invention. MODES FOR CARRYING OUT THE INVENTION The present invention shall be described in the following as a typical converted vessel that originally may be an Aframax type of vessel but where the vessel may be of any type of wide-body vessels typical of barge type utilised or constructed for the purpose or any commercial vessels typical like tanker, bulk-vessels and others suitable for conversion into a stable installation vessel capable to carry a multiple of windmill assemblies per offshore trip and by the fitted DP system or anchoring system or by a combination of these system are having a station keeping ability enabling the vessel to install complete windmill assemblies, with their foundation structures represented by typical four-leg jacket type of substructure but where the foundation structure may be of any type of sub structures, typical being jacket substructure, mono- or multi-pile structures, mono-tower or gravity type of substructures of steel or concrete material or anchored floating mono-tower type of substructure of steel or concrete material, with a reference to the attached descrip^¬ tion and drawings. The invention also relates to the method for performing the above installation.

Figure 1 is presenting a side view of such a converted vessel 9, in this case being an Aframax type of vessel fitted with side pontoons 47 made integrated into the sides of the hull 46 of the vessel 9 for stability reasons, showing five windmill assemblies 37 with their foundation structures represented by typical four-leg jacket substructures 34 and as a minimum typically supported for transport by hydrauli- cally driven support plates at the top of support structures 14 against the tower 22. The sixth windmill assembliy is shown being skidded to split bow 10 with its opening for preparation of installation. On the split bow 10 are shown two installation towers 11, one on each side of the split bow. These are fitted with the necessary installation mechanism to be connected to the windmill assembly ready to be low- ered to seabed. Shown on the seabed in front of the split bow are stick-ups 27 of the pre-installed four piles providing full stability when the four jacket legs 25 of the windmill assembly have been landed, and the legs locked and grouted onto the stick-ups. Depending on soil condition, an alternative to piling may be four similar stick-ups 32 mounted on a pre-installed suction-anchored template foundation. For the anchored floating mono-tower type of substructure, an alternative pre-laid anchoring system may be established on seabed, ready to be picked up by the vessel for connection to the mono-tower when this has been lowered into water on the split bow.

Figure 2 is presenting a side view with the first windmill assembly 37 with its foun- dation structure represented by a typical jacket substructure 34 already skidded onto split bow 10. Either bow or stern of the vessel 9 may be heading into the weather for maintaining position of the split-bow and windmill assembly above the installation point on the seabed during this installation operation.

The four jacket legs 25 are at the base penetrating skid shoes 7 and locked to shoes by an internal hydraulically actuated pile gripper 23 being an integrated part of the legs. This locking mechanism is essential during transport, skidding and eventually when landed onto pre-installed subsea stick-ups 27 on piles or on the alternative suction anchored foundation frame 32. Truss support bridges 17 may connect the two installation towers 11 on the bow, with the six sets of support structures 14 as required during transport and skidding. An articulated truss-work 16 including a releasable hoop iron on the windmill tower 22, is arranged on the side of the carriage 15, giving lateral support to the tower during skidding of the windmill assembly 37, as well as during lowering and rotating of the windmill.

During skidding to the installation towers in the split bow 10 of the vessel, the windmill assembly is supported at two levels, i.e. on the deck level by the skid- shoes 7 on skid beams 6 and at level of support bridges 17. The articulated truss- work 16 with the releasable hoop iron is connecting the windmill tower 22 to the support skid carriage 15which is vertically arranged on runway 18.

At deck level, the typical four-leg jacket substructure 34 in the windmill assembly 37 is supported and skidded on four skid shoes 7 running on two skid beams 6, enabling the assembly to be skidded towards vessel split bow 10. Two hydraulically driven skid units 35 are connected to two of the skid shoes, one unit on each skid beam. A similar hydraulic skid unit is connected to the supporting skid carriage 15 on runway 18 mentioned above for simultaneous skidding operations following the skidding on deck level and for giving lateral support of the tower 22 during these skidding operations. For the typical anchored floating mono-tower type of substruc- ture alternative, the mono-tower part may be locked to and skidded on one or more skid shoes centrally located on deck and running on one or more skid-beams, enabling the assembly to be skidded towards vessel split bow by means of one or more hydraulic driven skid units. Alternatively, only the upper part of the floating mono- tower may be transported, skidded and installed by being landed and installed onto a pre-installed and anchored lower part of floating mono-tower type of substructure.

Figure 3 is showing a front view on bow with the foundation structure of a windmill assembly 37 represented by typical jacket substructures 34 with jacket legs 25 lifted clear of skid shoes 7 and exposing pile grippers 23. The split bow 10 is allow- ing the windmill assembly to be lowered down through the opening of the split bow of the vessel 9 by means of an installation system that for this alternative shown on figure 3 comprises a minimum of four (600 T typical) strand bundle jacks 1, but may also consist of winch system alternatives as indicated on figure 7 and 8. The jacks 1 are located on the jacking decks 13 above the jacket in the two installation towers 11 made as truss-work, one on each side of jacket to be lowered and with minimum two strand bundle jacks on each. Each jack is installed on a support table 36 allowing the jack to align to the bundle direction to minimise the required force to rotate the jacket for final alignment of legs to stick-ups 27, 32. Four bundle reels 3 located above the jacks are arranged for storing of the four bundles 2. The four lift-point fixtures 4 on top of the jacket are located on the extended horizontal truss beams on the top of the jacket. Four (600 T typical) line compensators with accumulators 5 are connected to the lift points 4 and into the strand bundles 2, one in each bundle above the lift points to alleviate dynamic loads during lowering and landing of the windmill assembly 37. Other compensating means may be introduced below the jacks 1 as part of the support table 36 and / or on the stick- ups 27 on the piles, or on the suction anchored foundation frame 32 to keep the tension in the lowering bundles 2 and the vertical movement and impact of the windmill assembly within defined limitations and under proper control.

During lowering of the windmill assembly 37 it will be supported at two levels. The upper part, being the windmill tower 22, will be laterally supported by the articulated truss-work with releasable hoop iron 16 at the top of installation tower 11, while the jacket substructure 34 is laterally supported by two guide pins 20 on both sides of the jacket substructure. The pins are moving inside guide channels 19 located on the sides of the installation towers 11 and the sides of split-bow 10. The funnel at the lower end of the guide-channel will allow the jacket with guide pin to re-enter in case of a reversed operation is required.

For the typical anchored floating mono-tower type of substructure alternative, the mono-tower of the windmill assembly may be transported, skidded, suspended, lowered and guided into water in a similar method as outlined above for the jacket type of substructure. After having stabilised the windmill assembly in the water by ballasting the mono-tower, a pre-laid anchoring system may then be picked up by the vessel and connected to the mono-tower for subsequent de-ballasting of the mono-tower for final tensioning up of the anchoring system. Alternatively, an upper part above water of the floating mono-tower may be transported, skidded, sus- pended, lowered and guided onto a pre-installed and anchored lower part of mono- tower, for subsequent locking onto it while in floating condition.

Figure 4 is presenting the jacket anchor legs 25 of a windmill assembly 37 represented by typical four-leg jacket support structure 34, lowered to a few meters above primary and secondary pile stick-ups 28,30, with guide pin 20 out of contact with installation guides 19. At this stage the guidelines 38 have been connected to the primary and secondary fixtures 29,31 on the stick-ups and tensioned by guideline tensioners 21 to stabilise the windmill assembly 37 laterally, together with the articulated support truss hoop iron 16 at the top of installation tower 11. Prior to this, the guideline fixtures 29,31 that may consist of external detachable pile grip- pers fitted with pad eyes, have been locked to the stick-ups.

Depending on vessel-orientation when positioning the bow 10 with windmill assembly 37 above stick-ups 27 / 32, and as an alternative to rotate the vessel, the jacket substructure 34 and thereby the windmill assembly, may be rotated laterally in order to have the jacket legs aligned with stick-ups 27 or alternatively 32. Two (30 T) tagger winches 12 are considered required to rotate the jacket substructure while being suspended in the strand bundles / the lift and lowering wire line 2 / 39 just above the stick-ups, allowing about 15 degrees rotation in either direction as required. Four (30 T) tagger winches are arranged on a mezzanine deck in the in- stallation tower 11 available for this operation as required. Support tables 36 are arranged underneath the jacks 1 to allow the jacks to align with the direction of the bundles 2 during this rotation.

By means of the DP system fitted on the vessel, the vessel bow or stern is as a first step in aligning the windmill assembly on the bow to the seabed position, kept against the wind / waves with weather up to typically 30 degrees on the bow or stern, with the bow positioned in optimum position for landing typical jacket anchor legs 25 of a windmill assembly onto pre-installed stick-ups 27 / 32 situated either on pre-installed piles or on suction foundation frame, respectively.

As an alternative to further rotate the vessel beyond the typical 30 degrees on the weather direction, the tagger winches 12 may as required be used as a second step in aligning the jacket substructure 34 prior to landing jacking-legs 25 onto stick- ups. By the tagger-winches pulling and rotating the jacket the said 15 degrees and by adding the said 30 degrees, this is making it 45 degrees in total in a more unfavourable weather-direction scenario, enabling the deployment system to install most effectively in two of the four quadrants, while two of the quadrants will be somewhat less effective, depending on the actual weather direction.

For a third and last step, a minimum of two guidelines 38 with guideline tensioners 21 are planned to be used for fine alignment of the jacket anchor legs 25 when en- tering the legs onto the stick-ups 27 or 32. The guidelines are connected to guideline fixtures 29 / 31 on top of the stick-ups.

Once finally aligned, the windmill assembly 37 is to be lowered on the strand- bundles / lift wires 2 / 39 with a working remotely operated vehicle (WROV) moni- toring the first jacket anchor leg 25 entering the primary anchor pile stick-up 28, which is the highest of the stick-ups, and then the diagonally located jacket anchor leg entering the secondary anchor pile stick up 30, which is the next to the highest stick-up, followed by the last two legs entering the last two stick-ups.

Figure 5 is showing the windmill assembly 37 represented by the tower 22 sup- ported by a typical jacket substructure 34, lowered and locked onto stick-ups 27.

Locking of the windmill assembly's ja legs 25 to the skid shoes 7, as well as locking same to the subsea stick-ups 27, may be accomplished by the same hydraulic activated internal pile gripper 23 being an integrated part of the jacket legs 25 at base. The activation subsea may be accomplished either by WROV or from vessel control room. The stick-ups on the pre-installed piles or on the suction anchored foundation frame provide full stability when the windmill assembly is landed and locked to the pile by the internal pile grippers 23 in the jacket-legs 25, allowing the vessel to release all its connection to the windmill. The articulated truss work with hoop iron 16 is then released from the connection to the windmill tower 22 and relocated to the next tower to be skidded. Strand bundles / lift and lowering wire lines 2, 39, guidelines 38 and tagger winches 12 will then be disconnected. At this stage the vessel will pull partly out from position and connect up grouting connections to the stick- ups and start pumping grout into the connections to establish a permanent connection. Alternatively, this last operation of pumping grout may be carried out by an- other smaller vessel .

The stick-ups 27 may as required be designed as part of a hydraulic damper when the jacket leg 25 is entering the pile stick-ups by the effect of the jacket leg with a cone at the end for ease of entry is acting as a piston in a cylinder the latter represented by the stick-up, and by designing a proper throttling of the water exits from the stick-ups, a damping effect of the landing-impact may be achieved which may be important for the wind assembly impact capacities. In order to avoid an unwanted locking effect of the jacket-leg 25 at the entry of the primary stick-up 28, a step-wise reduction of the inner diameter towards the top of the stick-ups may assist in avoiding this effect.

Figure 6 is showing the articulated support truss with hoop iron 16 initially con- nected onto tower 22, now shown disconnected when jacket anchor legs 25 are locked onto pile stick-ups 27 or 32.

In figure 7 the strand bundle jacks 1, strand bundles 2 and tension compensators with accumulators 5 directly coupled into the lift string as presented in figure 3, are here replaced with an installation system alternative comprising four or more sets of winches 41 and tension compensators with accumulators 5 with running wire sheaves 40, all located on deck of vessel 9 and with the lift and lowering wire line 39 brought to lift point on top of the jacket structure 34 via suspended wire sheaves 40 in top of installation towers 11, so that the tension in the wire and the vertical movement and impact of the assembly 37 are kept within defined limits by said tension compensator 5 when installing a windmill assembly on seabed..

In figure 8 the strand bundle jacks 1, strand bundles 2 and tension compensators 5 directly coupled into the lift string as presented in figure 3, are here replaced with an installation system alternative comprising four or more power cylinders with accumulators 43 vertically supported on deck of vessel 9, with the rods connected to yokes 45 supported by vertical guides 42 inside the installation towers 11 and with four or more fixed and parallel lift and lowering wire lines 39, with one end fixed to an anchoring point 44 on deck and the other end at lift point fixture 4 on top of jacket and run over suspended sheaves 40 on the yokes, thereby transforming the push or pull from the power cylinders to upward or downward motion, respectively, on the jacket 34 and thereby also the windmill assembly 37, and by the connected accumulators in the hydraulic power system 43 allowing it to work as compensators, so that the tension in the lift and lowering lines 39 and the vertical movement of the assembly 37 are kept within defined limits when installing a windmill assembly on seabed. It will be understood that the invention is not limited to the exemplifying embodiments described herein, but may be varied or modified by the person skilled in the art within the scope of the claims. For instance, it is within the invention to use the method steps in reverse order to decommission and recover an earlier installed windmill a LISTING OF REFERENCE NUMERALS IN THE FIGURES

I . Strand Bundle Jacks (600 T typ.)

2. Strand Bundles

3. Strand Bundle-reels

4. Lift-point fixtures on jacket top structure

5. Line Compensators or Tensioner w/ accumulators (600 T typ.)

6. Skid-beams

7. Skid-shoes

8. Bundle-anchors

9. Converted Vessel-hull including side-pontoons

10. Split-bow with opening

I I . Installation-towers

12. Tagger Winches (30 T typ.)

13. Jacking-deck

14. Support-structures with hydraulic driven support-plates at top onto mill tower as part of sea fastening during transport phase.

15. Support Skid-carriage

16. Articulated Support-truss with a releasable hoop iron connected onto tower for support during skidding, lowering and rotation of Mill assembly.

17. Support-Bridges connecting the support-structures to installation towers as required.

18. Run-way for Support Skid-carriage

19. Installation Guide fitted with funnel for re-entry at lower end

20. Guide-pin on jacket substructure

21. Guideline Tensioners w/ accumulators and winches (the latter not

shown).

22. Tower of windmill assembly

23. Hydraulic Actuated internal pile grippers for locking of jacket's four anchor-legs to skid-shoes on deck and to stick ups on pre-installed subsea piles or suction anchored foundation frame.

24. Adjustable Guideline Sheaves.

25. Jacket's Anchor legs penetrating skid-shoes on deck and stick-ups sub sea.

26. Guideline Funnels on jacket legs.

27. Stick-ups on pre-installed piles

28. High level Primary Anchor Pile Stick-ups Primary Guideline Fixture on high level stick-up (External pile gripper w/ pad eye).

Next to the high level Secondary Anchor Pile Stick-up

Secondary Guideline Fixture on next the high level stick-up (External pile gripper w/ pad eye) .

Stick-ups on suction Anchored Foundation Frame

Bow Support Beam

Jacket substructure

Hydraulic Skid Unit

Support Table for Bundle Jack

Windmill Assembly

Guidelines

Lift

Sheaves

Winches

Vertical guides inside installation towers for guiding yokes

Power cylinders with accumulators

Wire line anchoring points

Yokes

Hull of the vessel

Pontoons

Claims

1. A method of relocating from an onshore assembly yard a number of complete windmill assemblies (37), each consisting of a tower (22) with generator fixed to a foundation substructure (34), to an offshore site and subsequent installation by a vessel (9) having a split bow (10), c h a r a c t e r i z e d i n t h a t the windmill assemblies (37) are skidded over the bow (10) onto the deck of the vessel while standing in an upright position and remaining in that position during transit to the offshore installation site, where each windmill assembly (37) is skidded to the split bow (10) and lowered through the split bow to the seabed by installation means (1-5, 11, 39-45) on the bow while the vessel (9) is kept on station at least partly by a dynamic positioning system, and in that the windmill assemblies (37) are guided and secured as necessary at upper and lower parts thereof in all operating scenarios including skidding, transport, lowering and any subsequent recovery for de-commissioning of an earlier installed windmill assembly by reversing the operating procedures above.

2. A method according to claim 1, wherein the vessel (9) transports the windmill assemblies (37) while supported by support structures (14) with hydraulic driven support means at the top against the windmill towers (22) and with anchor legs (25) via skid shoes (7) locked to skid beams (6) during transport to the offshore site, followed by skidding the windmill assemblies (37) one by one to a split bow with an opening (10) for, by means of an installation mechanism consisting of installation towers (11) with wires (2, 39) and tension compensators (5), lowering the assembly (37) through said opening (10) onto stick-ups (27), either on pre-installed piles or stick-ups located on a suction anchored foundation frame (32) on the seabed at said offshore site, so that the tension in the wires (2, 39) and the vertical movement of the assembly (37) are kept within defined limits by the tension compensators or by hydraulic damping effect designed into the stick- ups.

3. A method According to claim 1 or 2, wherein a vessel (9) is used that is fitted with dynamic positioning facilities capable of keeping the bow or stern against the wind and waves such that the split-bow with opening (10 ) is positioned on station above pre-installed stick-ups (27) for allowing lowering and landing the windmill assembly (37), with the possibility to maintain the bow positioned above the stick-ups with weather up to at least 30 degrees on the bow or stern, and with the possibility of subsequent rotation of the windmill assembly (37) by about 15 degrees when preparing for final landing of the foundation substructure of the windmill.

4. A method according to claim 1, 2 or 3, wherein said vessel is a converted vessel such as an Aframax or Suezmax type of vessel (9) having pontoons (47) made integrated into the sides of the hull (46) of the vessel, or the vessel may be of any type of wide body vessels suitable for conversion into a stable installation.

5. A method according to any one of claims 1-4, wherein the foundation substructures are jacket structures (34), and wherein the substructures (34) are fitted with two guide pins (20) following two installation guide channels (19) on two of the sides of the installation towers (11), giving lateral guiding of the jacket structure (34) down to the vessel bottom, whereupon further lowering of the substructure (34), lateral guiding of the windmill assembly (37) is achieved by a support skid carriage (15) connected with a articulated support truss with hoop iron (16) onto the tower (22), and with at least two guidelines (38) suspended and tensioned between two guideline tensioners (21) situated on the level of the vessel deck, and guideline fixtures (29, 31) of external detachable pile gripper type of fixture or a similar type of fixture locked onto the top of the stick-ups (28, 30).

6. A method according to any one of the preceding claims 2-5, wherein the anchor legs (25) at the base of the substructure (34) are fitted with integrated hydraulic actuated internal pile grippers (23), a locking mechanism locking the anchor legs to the skid-shoes (7) during transport as well as during skidding, and also locking the anchor legs (25) onto the subsea stick-ups (27) of pre-installed piles or suction anchored template, with subsequent grouting for a permanent connection.

7. A vessel for transportation of a plurality of windmill assemblies (37) from an offshore assembly yard to an offshore field for subsequent installation on the seabed, said vessel (9) having a split bow (10),

characterised in that the vessel (9) is fitted with skidding means (6, 7) to skid a plurality of windmill assemblies (37) from said yard over the stern onto storage positions on the deck of the vessel, in that as part of a sea fastening system the vessel (9) is fitted with support structures (14) for each windmill assembly (37), one such support structure being fitted with a hydraulically driven support mechanism at the top for connection onto windmill tower (22) for supporting the windmill tower (22) during transport, and in that the vessel is fitted with installation means (1-5, 11, 39-45) at the split bow (10) thereof.

8. A vessel according to claim 7, wherein said vessel is preferably a converted vessel made from an Aframax or Suezmax type of vessel (9) having been fitted with extra power generators making the vessel suitable for dynamic positioning and having pontoons (47) integrated in the sides of the hull (46) of the vessel, the converted vessel being of any type of wide-body vessel suitable for conversion into a stable installation vessel that may be provided with a split bow (10).

9. A vessel according to claim 7, wherein as part of a sea fastening system, skid beams (5) on the vessel (9) are enabling anchor legs (25) of a four leg jacket sub-structure (34), said anchor legs being fitted with integrated hydraulically actuated internal pile grippers (23), to lock the windmill assembly to the skid beams via skid shoes on the anchor legs during transport to the offshore site.

10. A vessel according to claim 9, wherein the skid beams permit the windmill assembly (37) to be skidded to the split bow (10) by connecting hydraulic skid units (35) to skid shoes (7) on the skid beams, and by also connecting similar hydraulic skid units to a support skid carriage (15) connected with an articulated support truss with hoop iron (16) onto the tower (22) and running on a runway (18) for giving lateral support of the top part of the windmill assembly during skidding to the split bow (10).

11. A vessel according to any one of claims 7-10, wherein said vessel (9) is fitted with DP facilities or anchor mooring or a combination of these methods enabling the vessel to keep bow or stern against the wind and waves with the ability to maintain the split bow (10) in a stable position above the installation point on the seabed with weather up to at least typical 30 degrees on the bow or stern, with the purpose to lower the windmill assembly (37) with its foundation structure through the split bow (10) and land it accurately and safely onto the intended location on the seabed.

12. A vessel according to any one of claims 7-10, wherein on the deck of the split bow of said vessel (9) is fitted an installation mechanism comprising two installation towers (11) with at least four strand bundle jacks (1) located on a jacking deck (13), associated strand bundles (2) and tension compensators (5) for the lowering of the substructure and windmill assembly (37) through said split bow (10) onto stick-ups (27), either on pre-installed piles or stick-ups located on a suction anchored foundation frame (32) on the seabed at said offshore site, so that the tension in the strand bundles (2) and the vertical movement and impact of the assembly (37) are kept within defined limits by the tension compensators (5) or as required, by other compensating means underneath the strand jacks (1), or by hydraulic damping effect designed into the stick-ups.

13. A vessel according to claim 12, wherein that the installation mechanism on the split bow (10) comprises strand bundle jacks (1) fitted on top of a series of support tables (36) on jacking deck (13) in top of installation towers (11) on said vessel (9) causing the jacks (1) to align to the bundle direction when the windmill assembly (37) with sub-structure (34) all pulled aside and rotated by means of at least two tagger winches (12) connected to the jacket substructure (34).

14. A vessel according to any one of claims 7-11, wherein the installation mechanism on the split bow (10) of the said vessel comprises four or more sets of winches (41) and tension compensators with accumulators (5) with running wire sheaves (40) located on deck of the vessel (9), and with lift and lowering wire line (39) brought to lift point on top of the jacket substructure (34) via suspended wire sheaves (40) in top of installation towers (11), so that the tension in the wire line and the vertical movement and impact of the assembly (37) are kept within defined limits by said tension compensator (5) during operations.

15. A vessel according to any one of claims 7-10, wherein that the installation mechanism on the split bow (10) of the vessel (9) comprises four or more power cylinders (43) with accumulators (43) vertically supported on deck of vessel (9), with the cylinder rods being connected to yokes (45) supported by vertical guides (42) inside the installation towers (11) and with four or more fixed and parallel lift and lowering wire lines (39), with one end fixed to an anchoring point (44) on deck and the other end at fixture (4) on top of jacket substructure (34) and run over suspended sheaves (40) on the yokes (45) thereby transforming the push or pull from the power cylinders (43) to upward or downward motion on the jacket substructure (34), respectively, and whereby the connected accumulators work as compensators so that the tension in the lift and lowering lines (39) and the vertical movement and impact of the assembly (37) are kept within defined limits.