BACKGROUND OF THE INVENTION

1. Field of the invention

[001] The present invention relates to a method for deploying objects, such as offshore wind turbine foundations or subsea installations, from a vessel. The invention also relates to such a vessel.

2. Description of the related art

[002] Offshore wind turbine foundations are typically installed using cargo barges and crane vessels working together. The cargo barges ship the foundations to a desired offshore site. At this site, the crane vessel, which is equipped with a heavy lift crane, lifts each foundation off the cargo barge and places it on the seabed.

[003] This method involves a number of disadvantages. The cargo barge supplying the crane vessel is typically slow which limits the number of foundations that can be placed per unit of time. Lifting each foundation from the barge and placing it on the seabed is a complex task, due to movement of the vessels and swinging of the cargo, which may lead to damage. An example of a crane vessel is disclosed in WO 2012/070937.

[004] Especially in winter time or in heavy seas with high waves, movement of the vessels and swinging of the cargo can be problematic. Therefore, the existing methods only allow for the installation of wind turbine foundations during summer time and/or quiet weather. This leads to limited operation times and thus higher costs.

[005] Wind turbine foundations can also be installed using jack-up vessels such as shown in WO 2012/175091. Although use of such vessels can reduce the influence of waves and weather, they typically operate more slowly, due to the time required to complete the jack-up operation. Also, they tend to have a higher centre of gravity, increasing the moment of forces on the vessel. Jack-up vessels can also only operate in relatively shallow waters.

[006] Installation or placement of heavy items, like foundations and subsea installations can also take place directly from a heavy-lift or semi-submersible vessel. Nevertheless, it will be understood that a vessel designed for navigation will be less stable for heavy lift operations than a jack-up vessel or dedicated crane vessel. For a heavy lift crane located on such a vessel, lifting upwards from a cargo deck located well above the water line will significantly increase the centre of gravity of the vessel. In rough weather, motion of the vessel and in particular the top of the crane, makes lifting extremely difficult.

[007] In light of the above, it would be desirable to provide solutions which at least partially overcome some of the inconveniences of the prior art.

SUMMARY OF THE INVENTION

[008] According to the invention there is provided a method for deploying an object, such as an off-shore wind turbine foundation or subsea installation, from a vessel, the method comprising: providing a vessel comprising a submersible cargo deck and a heavy lift crane, wherein the object is present on the cargo deck; submerging the cargo deck to a position below a level of the water; lifting the object from the submerged cargo deck using the heavy lift crane to a desired position overboard, at least partially below the level of the water; and releasing the object at the desired position.

[009] In this method, only a single vessel is required, because the vessel simultaneously has a cargo deck and a heavy lift crane. Logistics of the installation of e.g. wind turbine foundations is thus simplified. The cargo deck is large enough for wind turbine foundations, wind turbine transition pieces, wind turbine tower parts or other objects of equivalent size. The crane is large enough to lift these objects from at least across the entire width of its own cargo deck. The vessel may be a mono hull vessel.

[010] The cargo deck (or working deck) is submersible. The person skilled in the art will understand that this means that the cargo deck can be submerged until it is completely flooded, i.e. to a position below a level of the water. The cargo deck may also be raised to a position entirely above the water level. Preferably, the cargo deck can be lowered until at least 5 meters below the water level, more preferably at least 10 meters below the water level, more preferably 15 meters below the water level. The draught of the vessel is typically controlled by water ballast tanks in the hull of the vessel. The draught can be controlled between 5 meters to 28 meters.

[01 1 ] In order to be capable of carrying various kinds of cargo, the cargo deck is preferably substantially planar, meaning that it is planar apart from any necessary facilities such as fences or staircases. It is however not excluded that the cargo deck could be stepped i.e. have different levels that could be submerged sequentially as the vessel is lowered in the water. For the purpose of the present specification, the deployment zone will be defined as the part of the cargo deck from which objects can be lifted by the crane. It will be appreciated that other parts of the vessel may be submersible and/or carry cargo but will not fall within the definition of the deployment zone if the crane cannot lift from this location. The cargo deck may be equal to the deployment zone. Preferably the deployment zone area is more than 30% of the area of the cargo deck. All embodiments and features related to the submerging of the cargo deck also apply to the deployment zone. The deployment zone and possibly the entire cargo deck are also preferably unobstructed, i.e. have no deck above them for substantially their entire area such that also very high structures such as wind turbine foundations can be supported by the cargo deck.

[012] It will be appreciated that, when lifting an object from the submerged deployment zone, the object is not only lifted by the heavy lift crane, but also buoyed up by its partial immersion in the water (Archimedes principle). This can also increase the lift capacity of the vessel.

[013] The submersible cargo deck has the advantage that the objects, such as wind turbine foundations or subsea installations, are not subjected to the dynamic forces of the splash zone at the water surface, since they are already at or below the water level once the cargo deck has submerged. The impact on the object and on the crane is thus greatly reduced when the object is lifted from the cargo deck while the cargo deck is submerged, especially when the object itself remains (partially) under water as well. The water thus efficiently dampens any swinging motion of the object.

[014] The submerging of the cargo deck also improves the motion performance of the vessel as a whole. The movement is reduced because the area of the vessel crossing the water level is reduced, and the centre of mass is lowered with respect to the water level. This is important in combination with the lifting of the heavy object, since vessel motion creates additional loads on the crane, especially if the heavy lift crane is placed off-centre with respect to the centre line (a line connecting the bow and stern of the vessel).

[015] Using the present invention, the installation of wind turbine foundations or similar structures is made more convenient, safer and with less damage, since swinging of objects is reduced. Moreover, the present method can be applied in more severe weather conditions in which the usual method is not sufficient. The method can therefore be used throughout the entire year, which also makes the use of the vessel, equipment and personnel more cost-effective. In particular, it will be understood that deployment from a submerged cargo deck may be preferred during rough weather, while during fair weather the cargo deck may remain raised. This may shorten transit time between loading locations and deployment locations.

[016] In an embodiment, the cargo deck is an elongated area and the heavy lift crane is located at one end of the cargo deck. This has the advantage that the vessel may serve two different functionalities. Firstly, it is adapted to lift heavy objects from its submerged deployment zone, as part of the cargo deck. Secondly, if the crane is located at one end of the cargo deck, the vessel can serve as a floating dock. Because the cargo deck is a large unobstructed area, it allows for carrying large loads such as other ships or barges.

[017] In an embodiment, the desired position is at the seabed and the method further comprises lowering the object to the seabed. This is especially advantageous when the object is a wind turbine foundation or subsea installation. In certain embodiments, the crane will be able to lower objects to a depth of 400 meters below the water level or more.

[018] In an embodiment, a plurality of objects are initially present on the cargo deck and the method comprises sequentially deploying each of the objects. Typically, offshore wind turbines are installed in groups. Therefore it is advantageous to install a sequence of wind turbine foundations. Preferably the cargo deck can hold at least 10 Jacket type wind turbine foundations of 1500 tonnes with a foot print of 28 meters by 28 meters or 1 1 Monopile type wind turbine foundations of 2000 tonnes each.

[019] In an embodiment, the method comprises raising the cargo deck to a position above the level of the water and navigating the vessel from a first desired position after deploying a first object to a second desired location for deploying a second object. This is especially useful if the locations are far apart.

[020] In an embodiment, the method further comprises loading the object onto the cargo deck at a dock and navigating the vessel to the desired position. The journey from the dock can take place in the raised position with the cargo deck only being submerged at the desired deployment location if weather conditions so require. In this context it is particularly advantageous to use a vessel which has a good transit speed and large cargo deck area, combined with the presence of a heavy lift crane.

[021 ] In an embodiment, the object remains at least partially submerged while lifting. As discussed above, this reduces the impact on the object because it avoids breaking the splash zone. It will be understood that for this to be achieved, there should be no upstanding obstructions between the cargo deck and the desired location that would require the object to be lifted up out of the water.

[022] The invention is in particular applicable to objects having a mass of greater than 100 tonnes or 500 tonnes, or even objects having a mass greater than 1000 tonnes or even objects having a mass greater than 3000 tonnes. Still further, it will be understood that it is particularly applicable to objects that are not buoyant i.e. objects that cannot be floated from a submerged cargo deck but must be lifted from the deck to their desired position. In an embodiment, the object comprises a wind turbine foundation or subsea installation.

[023] According to an aspect of the invention, there is provided a vessel for the transportation of objects, such as off-shore wind turbine foundations or subsea installations, and for deployment of such objects to desired positions overboard, the vessel comprising: a bow and a stern; a submersible cargo deck located between the bow and the stern, the cargo deck comprising a deployment zone having an area of at least 1000 m2; and a heavy lift crane, adapted to lift the objects from the deployment zone while the deployment zone is submerged, and move the objects in an at least partially submerged state, to the desired position overboard. In this context, overboard is intended to refer to a position from which the object can be lowered into the water and, if required, to the sea bed. Although in general, overboard will refer to positions beyond the extent of the vessel hull, it is not excluded that the object may be lowered through a moonpool or similar opening through the vessel.

[024] The vessel is preferably adapted for the method as described herein. The vessel may have all features and advantages as described in the context of the method. The person skilled in the art will understand that this is not a jack-up vessel. Jack-up vessels require to place legs to the seafloor in order to be able to use any heavy lift cranes. In contrast, the present vessel is able to use its heavy lift crane without the need to lower any legs. This saves operational time and thus lowers costs.

[025] The present vessel is self-propelled and may have a typical cruising speed / maximum velocity of at least 13 knots. In an embodiment, the vessel comprises propulsion capacity sufficient for worldwide navigation, possibly excluding artic areas. Additionally, it may have thrusters or mooring equipment, sufficient for maintaining the vessel on station during deployment operations both in submerged and raised positions. .

[026] It will also be understood that vessels with submersible decks are known for transport of heavy floatable items. Ships, rigs, barges and the like may be transported on such semi- submersible vessels, whereby the vessel is initially ballasted to sink the cargo deck below the water level. Once the cargo is floated above the submersible deck, the vessel is then raised to lift the cargo out of the water. The vessel can then navigate to a new location with sea-handling similar to a conventional vessel (to the extent that the cargo is adequately secured). The cargo is unloaded be reversing the operation. A vessel of this type is the Dockwise Vanguard. Such known vessels have hitherto been used for buoyant loads that are capable of floating from the submersible deck once the vessel is sufficiently submerged. Nevertheless, existing vessels of this type have not been intended for lifting from and too the submersible deck and have thus not been provided with heavy lift cranes commensurate to the potential capacity of the vessel.

[027] In an embodiment, the cargo deck is an elongated area and the heavy lift crane is located at one end of the cargo deck, preferably at a bow side of the cargo deck.

[028] In an embodiment, the bow is adapted to remain unsubmerged during submergence of the cargo deck, and the heavy lift crane is located on a forecastle on the bow, preferably adjacent to the cargo deck. The operations bridge may be located on the forecastle and may also be positioned at least around the crane pedestal in order to improve visibility. It will also be understood that reference to the stern and bow sections is with respect to the normal direction of movement of the vessel and that nevertheless, similar objectives could be achieved with a non- submersible stern section supporting the crane.

[029] If the crane is located on the forecastle, the vessel can have a larger cargo deck area, thus allowing for carrying more cargo. The forecastle may extend across the full width of the vessel or the hull. The forecastle may have a forecastle deck, with a width of at least 30 meters, preferably at least 50 meters, and a length of at least 40 meters, preferably at least 60 meters.

The forecastle deck may be used for objects such as wind turbine transition pieces or relatively small wind turbine components. The forecastle deck is preferably reachable by the heavy lift crane over its entire area. The forecastle may comprise all accommodation for the vessel’s personnel. The advantage of having a large and especially wide, forecastle is that the area crossing the water level is relatively large, which increases the stability of the vessel even in the lowered condition when the cargo deck is submerged.

[030] In an embodiment, the heavy lift crane is located off-centre with respect to a centre line connecting a bow and a stern of the vessel, preferably adjacent to a port-side or a starboard-side of the vessel. This provides for increased reach of the crane overboard on this side. It will be understood that the lift capacity at maximum sideways reach will be increased in the case that the vessel is lowered.

[031 ] In an embodiment, the vessel further comprises at least one stern section, which is adapted to remain unsubmerged during submergence of the cargo deck. The stern section improves the stability of the vessel in a longitudinal direction. The one or more stern sections may have a combined minimum reserve buoyancy of at least 1.5%. The stern sections preferably have a waterline area of less than 1000 m2, more preferably less than 500 m2. In certain

circumstances, the stern section may be omitted completely.

[032] In an embodiment, the stern section comprises one or more tanks which can be displaced across the cargo deck or removed completely. The tanks can be placed at the most convenient positions depending on the particular objects to be shipped. The exact shape of the cargo deck area is thus variable. Variation of the cargo deck by movement of such tanks may be a constructional process to be carried out e.g. in port by external interaction or may be integral to the vessel i.e. the vessel may reconfigure itself using e.g. the heavy lift crane.

[033] In an embodiment, the heavy lift crane has a lift capacity of at least 1000 tonnes, preferably at least 2000 tonnes and more preferably at least 3000 tonnes. The radius of the heavy lift crane may be more than 50 meters, preferably about 70 meters. The height of the heavy lift crane may be more than 80 meters above the deployment zone, preferably about 105 meters above the deployment zone. Furthermore, although reference is made to a heavy lift crane, this is not intended to be limiting on any particular type of crane and it is also not excluded that more than one such crane may be present.

[034] The cargo deck and deployment zone may have a width which is substantially equal to the width of the hull of the vessel, i.e. which extends from a port-side to a starboard-side of the hull. Preferably, the cargo deck and the deployment zone extend over at least 80% of the width of the hull. The width of the cargo deck and deployment zone may be more than 30 meters, preferably more than 40 meters, more preferably more than 50 meters. The deployment zone has an area larger than 1000 m2, preferably larger than 2000 m2, more preferably larger than 2500 m2. The length of the cargo deck may be more than 80 meters, preferably more than 120 meters, more preferably about 150 meters. Preferably, the cargo deck extends along at least 50% of the length of the vessel, more preferably at least 70% of the length of the vessel. The cargo deck may have a maximum tonnage / maximum load of 48000 tonnes. The cargo deck is typically provided at a height of 13 meters above the bottom of the hull.

BRIEF DESCRIPTION OF THE DRAWINGS

[035] The features and advantages of the invention will be further appreciated upon reference to the following schematic drawings of a number of exemplary embodiments, in which corresponding reference symbols indicate corresponding parts.

[036] Figure 1 shows a vessel according to an embodiment;

[037] Figure 2A-D schematically show the vessel of Figure 1 which deploys an object according to an embodiment.

[038] The figures are for illustrative purposes only, and do not serve as a restriction on the scope or the protection as laid down by the claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[039] Figure 1 schematically shows a vessel 1 which is intended for performing the method according to the invention. The vessel 1 comprises a cargo deck 2 which is located amidships in between a forecastle 10 at the bow 11 and a stern section 15 adjacent to the stern 12. The cargo deck 2 extends from the port-side 13 to the starboard-side 14. A heavy lift crane 3, intended to lift objects from the cargo deck 2, is located on the forecastle 10. The heavy lift crane 3 is located off- centre with respect to a centre-line CL connecting the bow 11 and stern 12 of the vessel 1 , in this case adjacent to the starboard-side 14. The heavy lift crane 3 is located close to the bow end 20 of the cargo deck 2, such that it can reach over a substantial part of the cargo deck 2. The part of the cargo deck 2 that can be reached by the heavy lift crane 3 is referred to as the deployment zone 6.

[040] In the illustrated embodiment, the crane 3 has a lifting capacity of 3000 tonnes and a radius of operation of 70 metres. The submersible cargo deck 2 has a width of 56 metres and an overall length of 150 metres, whereby it will be understood that only the first ~70 metres closest to the crane 3 serves as the deployment zone 6 according to the invention. The forecastle 10 has a width of 56 metres and a length of 65 metres. The stern section 15 has a total horizontal cross- section of 470 m2. The total deadweight tonnage/capacity of the vessel 1 is 48000 tonnes..

[041 ] Figure 2A schematically shows the vessel 1 of Figure 1 , wherein the cargo deck 2 including objects 4, 4a - wind turbine foundations - extends above the water level WL.

[042] In Figure 2B the same vessel 1 is shown, but now the cargo deck 2 including deployment zone 6 is submerged and provided below the water level WL. Submerging of the cargo deck 2 may take place by filling of suitable ballast tanks in the hull (not shown). Only the forecastle 10 including the heavy lift crane 3, the stern section 15 and the objects 4, 4a partially extend above the water level WL. It will be understood that for certain smaller objects, the objects may be submerged entirely below the water level WL. When submerged, the movement of the vessel 1 is significantly improved, which is especially advantageous in severe weather conditions. Furthermore, the object 4 can be lifted more easily since the water provides buoyancy to the object 4.

[043] Figure 2C shows the vessel 1 where the heavy lift crane 3 has been employed to lift the object 4. The object 4 is lifted such that it can be moved freely but is still partially below the water level WL. In this way, swinging of the object 4 is reduced and lowering the object 4 involve less impact since the splash zone has already been passed by the object 4.

[044] Figure 2D shows the vessel 1 where the object 4 is lowered overboard at a lateral side of the vessel 1 . The heavy lift crane 3 positions the object 4 on the seabed 5.

[045] Once the object 4 has been deployed, the vessel 1 can proceed to lift and lower a subsequent object 4a from the cargo deck 2, more specifically from the deployment zone 6. This may take place at the same location or at a different location. For navigation to a different location, the vessel 1 may be raised by pumping water from the ballast tanks to lift the cargo deck 2 above the water level. Alternatively, depending on the distance and the weather conditions, the vessel 1 may be moved to the new location with the cargo deck 2 submerged.

[046] The invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. In particular, although the vessel has been described for unloading of cargo and objects it will be understood that it may also be used for loading or recovering objects by lifting them from the water to the submerged cargo deck and that the described and claimed method encompasses both alternatives.

[047] Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.