Field of the invention

The present invention relates to vessels floating in a body of water. More specifically, the invention concerns a floating wind power plant comprising a buoyancy body configured for being submerged in water and supporting an equipment unit extending above the water, and a ballast element connected to the buoyancy body via at least one spacer structure.

Background of the invention

The state of the art includes WO 2010/093253 Al , which discloses an offshore wind turbine plant having at least one floating body supporting a wind turbine. The floating body consists of a buoyancy body, a spacer and a ballast structure, and the floating body is connected to a steering arm which is further connected to a connecting structure having a turntable which is connected to anchor lines tied to seabed anchors. The state of the art also includes JP 2005180351 A, describing a floating wind power generating device capable of suppressing lateral movement for maintaining a stable attitude with respect to a wave surface when a force is applied in the lateral direction relative to water surface. A supporting column equipped with a main float in the bottom, penetrates through a sub-float so as to be freely vertically movable. When the supporting column is moved vertically on the water surface by a wave force without a lateral force being applied, the water surface wind power generating device vertically moves without being interrupted by the sub-float.

The present applicant has devised and embodied this invention in order to overcome certain shortcomings of the prior art and to obtain further advantages. Summary of the invention

The invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention.

It is thus provided a floating wind power plant comprising a buoyancy body configured for supporting an equipment unit, and a ballast element connected to the buoyancy body via at least one spacer structure, characterized by guiding means in the buoyancy body configured for movable interaction with the at least one spacer structure such that the ballast element and the buoyancy body are movable with respect to one another, and the buoyancy body has an elongated shape with a forward portion and an aft portion, and the ballast element has a corresponding forward portion and an aft portion. In one embodiment, each of the spacer structures is at one end connected to the ballast element and is at the other, free, end furnished with first stopping means configured for interaction with corresponding second stopping means on the buoyancy body. A connection element for one or more mooring lines is connected to the ballast element forward portion. The connection element comprises preferably a turret rotatably connected to the ballast element, whereby the floating plant is rotatable in a nominal horizontal plane.

The buoyancy body comprises ballastable watertight compartments, and the ballast element comprises ballastable watertight compartments.

In one embodiment, the floating plant further comprises an extension element connected to a forward spacer structure free end and having a length such that an upper end of the extension element extend above a nominal water level when the buoyancy body is submerged below said water level. The equipment unit is preferably supported by an aft portion of the buoyancy body.

In a preferred embodiment, the equipment unit comprises a wind turbine generator supported by a column, a lower part of said column being connected to the buoyancy body.

The main object of the invention is to provide a floating offshore wind turbine plant which can be fully assembled onshore, and where the commissioning phase for wind turbine also is completed onshore. Such degree of work completion for the unit achieved onshore result in significant cost reductions.

Another main object of the invention is to reduce cost through a combined and controlled utilisation of solid ballast material and water ballast. The design is thus carefully tuned to benefit from such favourable ballast scheme.

Compared to the prior art, cost is also reduced with the present invention through the ability to combine and control the utilisation of solid ballast material and water ballast. A systematic ballast scheme in combination with the other design features of the invention will result in a wind turbine plant design having a most effective righting moment capacity. Low cost effective righting moment capacity is a key asset for such type of installation, which generally will be heavily exposed to overturning forces.

The present invention entails a number of substantial advantages compared to the prior art. The present invention involves only one floating body. This one floating body acts as a support structure for one wind turbine generator only, and this simplification allows for some substantial advantages for the present invention. In the invention, a steering arm function is integrated as part of the floating body. This is achieved by having the buoyancy body as an extensively longitudinally shaped body. The wind turbine tower is positioned at the aft end of the buoyancy body. Further, the connecting structure for the mooring system is made integrated into the front end of the ballast structure situated underneath the buoyancy body. The steering arm functionality, which is required to effectively wind vane, is obtained through the longitudinally shaped buoyancy body providing adequate horizontal spacing between wind turbine position and anchor system connection position. The invention comprises retractable spacer columns or retractable framework structure. This is achieved by letting the spacer elements run through openings in the buoyancy body. Stopper mechanism for each spacer element secures the final positioning of the spacer.

Brief description of the drawings

These and other characteristics of the invention will be clear from the following description of preferential forms of embodiment, given as non-restrictive examples, with reference to the attached drawings wherein:

Figure 1 is a side view of a first embodiment of a floating wind turbine plant in an operating condition; Figure 2 is a view of section A-A of figure 1 ;

Figure 3 is a view of section B-B of figure 1 ;

Figures 4 - 6 illustrate different stages of a preferred construction sequence;

Figure 7 illustrates a stopper mechanism on the spacer structures;

Figure 8 illustrates a vertical section of the lower portion of the connecting structure according to the invention;

Figure 9 illustrates a vertical section of the upper portion of the connecting structure according to the invention;

Figure 10 illustrates a vertical section of a second embodiment of connecting structure according to the invention; Figure 11 is a view of section C-C in figure 10;

Figure 12 illustrates a floating wind turbine plant where the power and utility cables are partly contained on a drum which is situated on a moveable support structure located on forward part of buoyancy body; Figure 13 illustrates a vertical section of the buoyancy body;

Figures 14 is a perspective view of the plant with the ballast element in an extended position, showing the part of the plant which is configured for being submerged when the plant is in operation (i.e. the wind turbine column and generator are not shown); and

Figures 15 is a perspective view of the plant with the ballast element in a retracted position, showing the part of the plant which is configured for being submerged when the plant is in operation (i.e. the wind turbine column and generator are not shown);. Detailed description of a preferential embodiment

Figure 1 illustrates a first embodiment of the floating wind power plant, in an operational configuration floating in a body of water W. The floating plant comprises a buoyancy body 3 connected to a ballast element 5 via spacers 4a,b,c. The buoyancy body 3 supports a wind turbine generator la via a column lb and a connecting column 2. The column lb and connecting column 2 may be one continuous element. The floating plant is moored to the seabed (not shown) via mooring lines 7 connected to the ballast element 5 at a connection element 12. The floating plant is thus free to rotate about the connection element 12 and thus align itself according to prevailing wind and water currents, thus defining a plant forward part 9a and a plant aft part 9b. Reference number 8 indicates power export cables and utility cables.

The floating plant is configured for allowing a combined use of solid ballast material and water ballast in order to maintain floating stability and angle of heel for the plant within given acceptable limits for all conditions. Referring to figure 2, the geometry of the buoyancy body 3 is longitudinal in shape and the body contains several watertight compartments 36, defined by transverse bulkheads 37 and longitudinal bulkheads 38. According to well known principles, some of these compartments are used as water ballast tanks. Hence water pumps74 (see figure 13; not shown in figure 2), may occasionally move water from one ballast tank to another to adjust angle of heel as needed for the floating plant. Typically in cases with no wind forces acting on the turbine plant, ballast water is stored in one or more ballast tanks 36a located the aft part 9b. Typically in cases with maximum wind forces acting on the plant, ballast water is stored in ballast tanks 36b located in the forward part 9a. Hence, according to the present invention, the longitudinal centre of gravity for the ballast element 5 is typically positioned some distance forward, measured relative to the longitudinal centre of buoyancy for the buoyancy body 3. Figure 2 also shows openings 31a,b for the aft spacer columns 4a,b and an opening 32 for the forward spacer column 4c (see figure 1). Referring to figure 3, the ballast element 5 also has a forward portion9a' and an aft portion 9b', and comprises ballast compartments 136 made up by transverse bulkheads 137 and longitudinal bulkheads 138. Solid ballast material, e.g. in the form of finely crushed rock, is placed in designated ballast compartments 136 prior to transport and offshore installation of the plant. Pipes (not shown) are routed inside each spacer element (4a-c) and further on to ballast compartments (36). The pipes will allow for placement of rock material inside the ballast compartments and for simultaneous evacuation of water and/or air from these compartments. Ballast operations are performed based on access provided at the upper end of spacer (not shown). Figure 3 also shows aft footings 51a,b for the aft spacer structures 4a,b and a forward footing 52 for the forward spacer structure 4c. Reference number 53 indicates an opening for the mooring line connecting element inside forward spacer structure(s).

Figure 4 shows the plant in a completed state at a fabrication facility, resting on a surface F. The spacer structures 4a,b,c run through each respective opening 31a,b, 32 (see figure 2) in the buoyancy body 3. Thus, during the construction and assembly phases for the plant, the buoyancy body 3 is resting on the ballast element 5. When the plant is floating in the water W, e.g. during tow-out (see figure 5) the ballast element 5 may remain in the retracted position with respect to the buoyancy body 3. The spacer structures conveniently comprises releasable locking means (not shown) to prevent accidental lowering of the ballast element 5. At a convenient time prior to or during installation, the spacer structures 4a-c are extended from the buoyancy body 3, thus lowering the ballast element 5. Flexible tension element 6, e.g. wires, may conveniently be connected between the buoyancy body and ballast element as indicated in figure 6, in order to rigidly connect the ballast element to the buoyancy body.

The spacer structures 4a-c are connected at a respective first end to the ballast element 5 (cf. description of footings with respect to figure 3, above). The spacer structures are movable with respect to the buoyancy body 3 in that they are slidably arranged in respective openings 31a,b, 32 as described above with reference to figure 2. Each of the spacer structures furthermore comprises a stopper mechanism at each respective free end. Referring to figure 7, the stopper mechanism comprises a conically shaped end portion 14 on each spacer structure and a corresponding conically shaped recess in the respective opening 31a,b; 32 in the buoyancy body 3. Each stopper mechanism is conveniently furnished with locking means, such as locking pins, welded brackets, etc. (not shown).

The forward spacer structure 4c furthermore comprises an extension 4d, which will extend above the nominal water level S when the floating plant is in a submerged operating condition (see e.g. figure 1). The purpose of a spacer extension 4d is to provide easy and dry access to the slewing ring system (described below) during installation and later service interventions. The spacer extension 4d is a slender pipe structure, optionally stiffened by vertical tension elements which are supported by several horizontal beams (not shown) cantilevered in radial directions at the top of the forward spacer element 4c. Referring to figure 1 and figure 8, the plant is secured to the seabed via mooring lines 7 connected to a connection element 12 in the ballast element 5. The connection element 12 comprises rotating body (turret) 17 which is connected to the ballast element 5 via bearings 44 and rotatable about a central member 16 which is anchored to the sea bed by means of a suitable anchor system. The system is positioned in the forward part of the ballast element 5 and slightly reaching out underneath the bottom of the element.

Connection points 19 for the mooring lines 7 are included in the central member 16 of the turret 17. The central member 16 further includes at least one guide tube 20 intended for high voltage power cables and utility cables 8 to be routed between the wind turbine plant and the seabed.

An alternative solution for the connecting element is to include guide tubes 21 for the mooring lines 7 (see figure 10). Hence, for this alternative, the central member 1 lb of the turret holds multiple guide tubes 21. At least one centrally located guide tube 21 ' contains high voltage cables and additional cables 8. In addition, the connecting structure contains one guide tube 21 for each mooring line 7.

Figure 11 shows one centrally located guide tube 2Γ for power and utility cables surrounded by four tubes 21 each designated for a respective mooring line. Fewer or more tubes may be used, according to the requirements in each specific case. The anchor line guide tubes run from the base of the ballast structure through the spacer and further to the top of the spacer extension, as shown in figure 10. An upper axial bearing 62 and a lower radial bearing 63 are provided. At the upper end of each anchor line guide tube inside spacer extension is located a mechanism for secure fastening of anchor line (not shown). Similarly the guide tube allocated for electrical cables run through the turret to the top of the spacer extension. At this top elevation is mounted the exchangeable utility box 61 containing the slewing ring for high voltage power transfer and the signal transfer system required for operation and control of the installation, see figure 9 and figure 10 respectively. The forward spacer column is mounted onto the top of the ballast structure and aligned with the connecting structure located inside the ballast structure. In the case of spacer in the form of a retractable framework structure, the elements being part of the connecting structure is located inside forward vertical framework leg. In the case of spacer in the form of closed pipe structure, connecting structure will be inside forward closed pipe structure. A stairway and platform (not shown) for offshore access to the wind turbine plant, is located on the aft end of the buoyancy body. Due to the wind vane functionality, inherent for the invention, the stair and platform arrangement will always be positioned on the favourable leeward side. The risk of damage, due to extreme waves hitting the installation, is reduced. Further, the solution according to present invention, allow service personnel to access the wind turbine plant in larger wave heights than would be possible from the opposite side which faces the incoming wind generated waves.

The present invention involves a method of assembly and installation of the wind turbine plant. The wind turbine plant is assembled inshore, in a dry dock, on a barge, or on land (F, see figure 4) for later skidding onto a barge for putting afloat or onto skid beams which have been extended into sufficiently deep water.

The invention involves utilisation of solid ballast and water ballast in combination with the design carefully tuned to benefit from a ballast and trim scheme which combines the use of these two types of ballast material.

Buoyancy provided by the buoyancy body 3 and the ballast structure brings the unit afloat (see figure 5). The unit is moved to a site having sufficient draught for lowering of ballast structure to operating position. Water ballast is filled into the ballast structure as required to lower the ballast structure and the attached spacer to operating position (see figure 6). Following the fixation of spacer structures 4a-c to the buoyancy body 3 and pre-stressing of tension elements (6), the ballast operations proceed. Ballast material in the form of finely crushed rock material is placed in the ballast element 5, utilising transport pipes installed inside the spacer structures. Following the completed rock ballast operation and the corresponding compensating water ballasting as indicated above, additional water is filled into water ballast tank inside the buoyancy body. This additional ballasting is in order to submerge the buoyancy body to operational depth (see figure 1), and involves filling of water tanks in the buoyancy body. The present invention involves method of connecting the wind turbine plant to suitable mooring lines 7, and to connect cables 8 which are linked to a substation during operation of the plant. For the disconnection of the turbine plant, the operations listed below will need to be carried out in the reversed order.

With the floating body positioned at operational draught, the plant is towed offshore by a tug to given location within the wind park, and hooked-up to pre-installed mooring lines. The mooring system are attached to the connecting structure in the forward part of the ballast structure. For the present invention one alternative for the installation operation is to have mooring line segments 7 pre-connected to the designated fastenings points located in the central part of the turret (cf. figure 8). When the wind turbine plant is positioned at location, the pre-installed anchor line segments are fixed to corresponding anchor line segments which have been pre-installed. Following completed anchoring of the plant, the power and utility cables 8 are pulled through the guide tube located in central part of turret. The guide tube provided for electrical cables run through the turret to the top of the spacer extension reaching above still water level (cf. figure 9). At this top elevation is mounted the exchangeable utility box containing the slewing ring for high voltage power transfer and the signal transfer required for operation and control of the wind turbine plant.

Another alternative for the installation operation is to pull up the anchor line segments through guide tubes 21 for anchor lines (see figure 10). Hence, for this alternative solution the central member 1 lb of the turret holds multiple guide tubes containing pre-installed pull lines. When the wind turbine plant is positioned at location, the pre-installed anchor line segments are fixed to the corresponding pre- installed pull line segments. Winch on service vessel is used in order to reel the pull line and bring the anchor line inside the connecting structure for fixation to the wind turbine plant. Following completed anchoring of the plant, the power and utility cables are pulled through the guide tube located in central part of turret.

For the present invention another solution for the power and utility cables is to have the cables pre-installed along with a pre-installed utility box required for high voltage power transfer and signal transfer. Cables as required to link to substation may be spooled on a drum 45 located on a temporary and removable support frame 46 attached to the buoyancy body 3 and which also provide access for personnel during installation, as shown in figure 12.

When the plant has been connected to the mooring system in the offshore wind park, water ballast may be removed from the buoyancy body for the case where a pre-stressed mooring system is to be used at given location.