Patent Publication Number: US-2023159141-A1

Title: Method of Assembling and Deploying a Floating Offshore Wind Turbine Platform

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to wind turbine platforms. In particular, this invention relates to an improved method of assembling and deploying a floating offshore wind turbine (FOWT) platform into a body of water. 
     Wind turbines for converting wind energy to electrical power are known and provide an alternative energy source for power companies. On land, large groups of wind turbines, often numbering in the hundreds of wind turbines, may be placed together in one geographic area. These large groups of wind turbines can generate undesirably high levels of noise and may be viewed as aesthetically unpleasing. An optimum flow of air may not be available to these land-base wind turbines due to obstacles such as hills, woods, and buildings. 
     Groups of wind turbines may also be located offshore, but near the coast at locations where water depths allow the wind turbines to be fixedly attached to a foundation on the seabed. Over the ocean, the flow of air to the wind turbines is not likely to be disturbed by the presence of various obstacles (i.e., as hills, woods, and buildings) resulting in higher mean wind speeds and more power. The foundations required to attach wind turbines to the seabed at these near-coast locations are relatively expensive, and can only be accomplished at relatively shallow depths, such as a depth of up to about 45 meters. 
     The U.S. National Renewable Energy Laboratory has determined that winds off the U.S. Coastline over water having depths of 30 meters or greater have an energy capacity of about 3,200 TWh/yr. This is equivalent to about 90 percent of the total U.S. energy use of about 3,500 TWh/yr. The majority of the offshore wind resource resides between 37 and 93 kilometers offshore where the water is over 60 meters deep. Fixed foundations for wind turbines in such deep water are likely not economically feasible. This limitation has led to the development of floating platforms for wind turbines. Known floating wind turbine platforms are formed steel and are based on technology developed by the offshore oil and gas industry. There remains a need in the art however, for improved methods of assembling and deploying a FOWT platform. 
     SUMMARY OF THE INVENTION 
     This invention relates in general to methods of assembling and deploying a floating offshore wind turbine (FOWT) platforms and the wind turbines mounted thereon. The FOWT platforms described herein are characterized by a negatively buoyant mass suspended from a positively buoyant floater by a plurality of suspension lines. In particular, this invention relates to an improved method of assembling and deploying a floating offshore wind turbine (FOWT) platform including floating a hollow outer tank in a floating assembly area of a body of water, the hollow outer tank having transit lines and suspension lines attached thereto, floating a buoyant floater in the floating assembly area of the body of water, and placing permanent ballast material in the outer tank to define a mass, and sinking the mass to a seabed of the body of water. Free ends of the transit lines and the suspension lines are raised to a surface of the body of water with buoys, and the buoyant floater is moved to a position over the mass. The transit lines are attached to a lifting device in the buoyant floater and the suspension lines are attached to a portion of the buoyant floater, the combined buoyant floater and the mass defining a FOWT platform. The mass is lifted with the lifting device to a point directly under the buoyant floater, and the FOWT platform is towed to an installation site in the body of water. Mooring lines are attached to anchors in the seabed and to the buoyant floater. The mass is lowered with the transit lines and the lifting device to a depth wherein the suspension lines are taught, thus suspending the mass with the suspension lines to define a suspended mass. The transit lines are then stored or removed from the mass. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a floating offshore wind turbine platform assembled and deployed according to the improved method of this invention. 
         FIG.  2    is an alternate perspective view of the FOWT platform illustrated in  FIG.  1    showing the suspended mass and the suspension lines connecting the suspended mass to the buoyant floater. 
         FIG.  3    is a cross-sectional view of the suspended mass illustrated in  FIGS.  1  and  2   . 
         FIG.  4 A  is a schematic illustration of a first step of a first embodiment of the improved method of this invention. 
         FIG.  4 B  is a schematic illustration of a second step of the first embodiment of the improved method of this invention. 
         FIG.  4 C  is a schematic illustration of a third step of the first embodiment of the improved method of this invention. 
         FIG.  4 D  is a schematic illustration of a fourth step of the first embodiment of the improved method of this invention. 
         FIG.  4 E  is a schematic illustration of a fifth step of the first embodiment of the improved method of this invention. 
         FIG.  4 F  is a schematic illustration of a sixth step of the first embodiment of the improved method of this invention. 
         FIG.  5    is a top plan view of a portion of the buoyant floater illustrated in  FIGS.  1  through  3   . 
         FIG.  6    is a perspective view of an alternate embodiment of the suspended mass illustrated in  FIGS.  1  through  3   . 
         FIG.  7    is a perspective view of the buoyant floater sub-assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described with occasional reference to the specific embodiments of the invention. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     Referring to the drawings, particularly to  FIGS.  1  and  2   , an embodiment of a floating offshore wind turbine (FOWT) foundation or platform is shown at  10 . The illustrated FOWT platform  10  is shown anchored to a bed of a body of water BW. The FOWT platform  10  is representative of a FOWT platform that has been assembled and deployed in accordance with the improved method of this invention. In the illustrated embodiment, the FOWT platform  10  is shown anchored to the seabed S. It will be understood that the seabed S may be the bed of any body of water in which the FOWT platform  10  will be placed into operation. 
     The illustrated FOWT platform  10  includes a buoyant floater  12  and a negatively buoyant mass  36  suspended from the buoyant floater  12 . The buoyant floater  12  supports a tower  14 , described below in detail. The tower  14  supports a wind turbine  16 . The FOWT platform  10  is structured and configured to float, partially submerged, in a body of water. Accordingly, a portion of the buoyant floater  12  will be above water when the buoyant floater  12  is floating in the water and deployed at an operational draft, described below. As best shown in  FIG.  1   , a portion of the buoyant floater  12  is also below the waterline, as illustrated by the line WL on a central column  26 , described below. As used herein, the waterline WL is defined as the approximate line where the surface of the water meets the FOWT platform  10 . One or more mooring lines  18 , two of which are shown in  FIG.  1   , may be attached to the FOWT platform  10  and further attached to anchors, such as the anchors  20  in the seabed S to limit to movement of the FOWT platform  10  on the body of water BW. 
     The illustrated buoyant floater  12  is formed from four pontoons  22  that extend radially outwardly from a central hub  24  and provide buoyancy. It will be understood that the buoyant floater  12  is configured to provide the primary source of buoyancy for the FOWT platform  10 . When assembled together, the pontoons  22  and the central hub  24  define a generally cross-shaped base  25 . The central column  26  is mounted to the central hub  24  and extends outwardly (upwardly when viewing  FIGS.  1  and  2   ) and perpendicularly to the pontoons  22 , and also provides buoyancy. Additionally, the central column  26  supports the tower  14 , which is attached thereto. Optionally, braces  28  may connect distal ends of the pontoons  22  to an upper portion of the central column  26 . Each pontoon  22  may be attached to one of the anchors  20  with one of the mooring lines  18 . 
     The illustrated pontoons  22  have a rectangular cross-sectional shape. Alternatively, the pontoons  22  may have other shapes, such as, but not limited to, cylindrical, conical, and tubular having other desired geometric cross-sectional shapes such as pentagonal and hexagonal. 
     If desired, a catwalk or access platform (not shown) may be attached to the upper portion of the central column  26 . Additionally, one or more access ladders (not shown) may mounted internally or externally of the central column  26 . 
     In the embodiments illustrated herein, the wind turbine  16  is a horizontal-axis wind turbine. The size of the wind turbine  16  will vary based on the wind conditions at the location where the FOWT platform  10  is anchored and the desired power output. Advantageously, it has been shown that the illustrated FOWT platform  10  is ideally suited to support conventional commercial offshore wind turbines  16  having an output in the range of about 6 MW to about 20 MW. Alternatively, the FOWT platform  10  may be configured to support wind turbines  16  may having an output less than about 6 MW and greater than about 20 MW. 
     The wind turbine  16  includes a rotatable hub  30 . At least one rotor blade  32  is coupled to and extends outward from the hub  30 . The hub  30  is rotatably coupled to an electric generator (not shown). The electric generator may be coupled via a transformer (not shown) and an underwater power cable (not shown), as shown in  FIG.  1   , to a power grid (not shown). In the illustrated embodiment, the rotor has three rotor blades  32 . In other embodiments, the rotor may have more or less than three rotor blades  32 . A nacelle  34  is attached to the wind turbine  16  opposite the hub  30 . In the embodiments illustrated herein, the wind turbine  16  is a horizontal-axis wind turbine (HAWT). Alternatively, the FOWT platform  10  may be configured to have a vertical-axis wind turbine (VAWT) mounted thereon. 
     Referring again to  FIG.  1   , the negatively buoyant mass  36  is shown suspended from the buoyant floater  12  by a plurality of suspension lines  38 . Advantageously, the mass  36  has a simple design including a housing or outer tank  40  having cylindrical wall  42  enclosed by a first axial end wall  44  (the upper wall when viewing  FIGS.  1  through  3   ) and a second axial end wall  46  (the lower wall when viewing  FIGS.  1  through  3   ). The illustrated outer tank  40  is preferably formed from reinforced concrete and has a diameter of 15.15 m and a height of 11.94 m. An interior of the outer tank  40  includes a permanent ballast space configured to hold permanent ballast material  48  in a lower portion thereof, and a variable ballast space  50  in an upper portion thereof. As best shown in  FIG.  2   , four fairleads  52  are mounted to the first end wall  44 . Alternatively, the outer tank  40  may have more than four fairleads  52 , may have three fairleads  52 , or may have two fairleads  52 . Additionally, the first end wall  44  may have only one, centrally mounted fairlead  52 , as shown in  FIG.  6   . The illustrated mass  36  has a cylindrical shape. Alternatively, the mass  36  may have other shapes, such as spherical, and cylindrical having a semi-spherical lower portion. 
     The permanent ballast material  48  may be any desired material, such as iron ore, or other material selected to achieve a desired, pre-determined mass necessary to balance gravity and the buoyancy of the assembled FOWT platform  10  as a complete system. 
     The plurality of suspension lines  38  that attach the mass  36  to the buoyant floater  12  may be formed from synthetic ropes, chains, cables, such as steel cables, and tubular steel structures. As shown in  FIG.  2    the mass  36  is attached to each pontoon  22  by two suspension lines  38 , for a total of eight suspension lines  38 . The mass  36  may also be attached to each pontoon by a single suspension line  38  or by more than two suspension lines  38 . When the FOWT platform  10  is fully deployed in a body of water BW, as described in detail below, each suspension line  38  will have a length sufficient so that the mass  36  will be suspended about 75.10 m below the buoyant floater  12 . The mass  36  may include a pump (not shown) for removing sea water from the variable ballast space  50 . 
     The distance that the mass  36  may be suspended below the buoyant floater  12  will vary based on the size of the buoyant floater  12  and the size of the wind turbine  16  and the tower  14  supported thereon. For example, in an alternate embodiment of the FOWT platform  10 , the mass  36  may be suspended about 40 m below a lower surface of the buoyant floater  12 . 
     The illustrated central hub  24  is hollow and has four side walls, each side wall having a width of about 10.98 m and an axial length or height of about 8.53 m. Each side wall of the central hub  24  defines a substantially vertical connection face to which the pontoons  22  will be attached. In the illustrated embodiment, the central hub  24  includes four side walls and has a substantially square cross-sectional shape. The four side walls are enclosed by a first axial end wall  24 A (the upper wall when viewing  FIGS.  1  and  2   ) and a second axial end wall (the lower wall when viewing  FIGS.  1  and  2   ). Alternatively, the central hub  24  may have other configurations, such as three side walls for the attachment of three pontoons  22 . The central hub  24  may also have internal watertight bulkheads for strength and/or to define variable water ballast chambers (not shown). 
     The illustrated central hub  24  may be formed from pre-stressed reinforced concrete, and may include an internal central cavity (not shown). Any desired process may be used to manufacture the central hub  24 , such as a spun concrete process, with conventional concrete forms, or with reusable concrete forms in a semi-automated process such as used in the precast concrete industry. The concrete of the central hub  24  may be reinforced with any conventional reinforcement material, such as high tensile steel cable and high tensile steel reinforcement bars or REBAR. Alternatively, the central hub  24  may be formed from FRP, steel, or combinations of pre-stressed reinforced concrete, FRP, and steel. The central hub  24  may be formed in sections, as described below. 
     Each pontoon  22  is also hollow and has a width of 10.98 m, a height of 8.53 m, and a length of 27.08 m. Like the central hub  24 , the illustrated pontoons  22  are formed from pre-stressed reinforced concrete as described above. Alternatively, the pontoons  22  may be formed from FRP, steel, or combinations of pre-stressed reinforced concrete, FRP, and steel. The pontoons  22  may be formed in sections, as described below. 
     The central column  26  is hollow and has a diameter of 10.44 m and a height of 26.5 m. The central column  26  includes a cylindrical side wall  27  having an outer surface, a first axial end wall  26 A (the upper wall when viewing  FIGS.  1  and  2   ), a second axial end wall  26 B (the lower wall when viewing  FIGS.  1  and  2   ), and defines a hollow interior space (not shown). The hollow central column  26  may include transverse bulkheads or decks configured and positioned for strength or for the mounting of electrical and mechanical components of the FOWT platform  10 . 
     Like the central hub  24  and the pontoons  22 , the illustrated central column  26  is formed from pre-stressed reinforced concrete as described above. Alternatively, the central column  26  may be formed from FRP, steel, or combinations of pre-stressed reinforced concrete, FRP, and steel. The central column  26  may be formed in sections, as described below. The illustrated braces  28  are formed from steel. 
     The pontoons  22  provide a source of buoyancy and define a water-plane area when the buoyant floater  12  is being towed in a body of water BW. Although not illustrated, each pontoon  22  may include watertight transverse bulkheads for watertight stability, and longitudinally extending bulkheads, and decks, arranged perpendicularly to side walls of the pontoon  22  for strength. Additionally, the internal bulkheads in each pontoon  22  may define ballast chambers (not shown) for variable water ballast. 
     The illustrated FOWT platform  10  includes four pontoons  22 . It will be understood however, that the FOWT platform  10  assembled in accordance with the improved method of this invention, may be constructed with three pontoons  22  or with more than four pontoons  22 . 
     Advantageously, temporary transit lines  56  and a lifting devices such as winches or chain jacks, schematically illustrated at  58 , movably connect the mass  36  to the buoyant floater  12  during deployment and re-deployment, as described below. The transit lines  56  may be lengths of chain or cables, such as steel cables. As best shown in  FIGS.  4 E and  5   , the transit lines  56  connect the mass  36  to the buoyant floater  12  via the winches or chain jacks  58 . The chain jacks  58  may be mounted at any desired location on or within the buoyant floater  12 , such as on the first axial end wall  24 A of the central hub  24 , as shown in  FIG.  5   . Alternatively, the chain jacks  58  may be mounted within the central hub  24 , or on or within one or more of the pontoons  22 . 
     As described above each of the components of the mass  36  and the buoyant floater  12 , i.e., the central hub  24 , the pontoons  22 , and the central column  26  may be formed from concrete and may have cross-sectional shapes other than as illustrated. 
     The mass  36  and the buoyant floater  12 , including the individual components of the buoyant floater  12 , may be formed in different sizes to be determined by the size of the wind turbine  16  and the tower  14  supported thereon. 
     In a first embodiment of a method of forming the mass  36  and the buoyant floater  12 , the outer tank  40  may be formed, i.e., pre-cast, as one large, monolithic section, or may be formed in two or more sections (not shown). Such sections may then be post-tensioned together during assembly of the outer tank  40 . Similarly, the central hub  24  may be formed as one large, monolithic section, or may be formed in two or more sections (not shown), although preferably not more than four sections. Such sections may then be transversely post-tensioned together (i.e., transversely to its axial length) during assembly of the central hub  24 . 
     The pontoons  22  may be formed in sections, such as sections having a length of about 3 meters. Thus, in one exemplary embodiment, each pontoon  22  may be formed in nine sections. As will be described below, the pontoons  22  may be longitudinally post-tensioned in pairs after each of the pair of pontoons  22  has been assembled to the central hub  24 . 
     The central column  26  may likewise be formed in sections, such as sections having a length of about 3 meters. Thus, in one exemplary embodiment, the central column  26  may be formed in about nine sections. As will be described below, the central column  26  may be longitudinally post-tensioned. 
     A first embodiment of a method of assembling and deploying the buoyant floater  12  includes forming and assembling a plurality of the pontoons  22 , the central hub  24 , the central column  26 , the mass  36 , and the braces  28  by any of the methods described herein in an on-shore location. Two of the pontoons  22  may then be assembled to the central hub  24  and the assembled pontoons  22  and central hub  24  may be post-tensioned from a distal end of one of the pontoons  22  to a distal end of the other of the pontoons  22 . A third pontoon  22  and a fourth pontoon  22  may then be assembled to the central hub  24  and may also be post-tensioned from a distal end of the third pontoon  22  to a distal end of the fourth pontoon  22 . 
     The central column  26  may then be assembled from sections on the first axial end wall  24 A of the central hub  24  and longitudinally post-tensioned. The braces  28 , if provided, may then be attached between the pontoons  22  and the central column  26 . The tower  14  and the wind turbine  16  may then be installed, thus defining the buoyant floater  12 . The buoyant floater  12  may be launched into the body of water BW by any conventional method, such as by using finger piers (not shown). 
     For example, a pair of finger piers (not shown) may extend outwardly from the shoreline SL or a dock (not shown). The buoyant floater  12  may be moved, such as by rail or other desired means of transport, onto the finger piers such that a portion of the buoyant floater  12  is supported on distal ends of both of the finger piers and supported thereon above a surface of the body water BW, and a portion of the buoyant floater  12  remains supported on the shore line or dock from which the finger piers extend. A floating launch platform, such as a semi-submersible or launch barge (not shown) may be moved between the finger piers and underneath the buoyant floater  12 . Once positioned beneath the buoyant floater  12 , ballast may be removed from the launch platform to cause the launch platform to rise in the body of water BW until the launch platform lifts the buoyant floater  12  off of the finger piers and the shoreline, thereby transferring the buoyant floater  12  onto the launch platform. The launch platform may then be towed to a launch area in the body of water BW. 
     Although the first embodiment of the method assembling and deploying the buoyant floater  12  is described as occurring in an on-shore location, it will be understood that the buoyant floater  12  may also be assembled in a dry dock, if the dry dock is large enough accommodate assembled buoyant floater or any of its component part or subassemblies, such as an assembled assembly of two pontoons  22  and the central hub  24 . 
     A second embodiment of the method of assembling and deploying the buoyant floater  12  occurs within a dry dock (not shown) and includes forming and assembling two of the pontoons  22 , assembling the central hub  24 , assembling the pontoons  22  to the central hub  24 , and post-tensioned the assembled pontoons  22  and central hub  24  from a distal end of one of the pontoons  22  to a distal end of the other of the pontoons  22  to define a buoyant floater sub-assembly  60  that is fully capable of floating on its own. 
     Once assembled, the buoyant floater sub-assembly  60  may be launched from the dry dock in a conventional manner and allowed to float in a floating assembly area (not shown), preferably near the dry dock. 
     Two additional pontoons  22  may then be assembled in the dry dock, and temporarily post-tensioned to provide structural integrity of the pontoons  22  prior to being assembled onto the buoyant floater sub-assembly  60 . Once assembled, the two additional pontoons  22  may be launched from the dry dock in a conventional manner and allowed to float in the floating assembly area (not shown) wherein the buoyant floater sub-assembly  60  is located. 
     Each of the two additional pontoons  22  are then mated to the open side walls of the central hub  24  of the buoyant floater sub-assembly  60  and attached thereto. The two additional pontoons  22  and the central hub  24  are then post-tensioned from a distal end of one of the pontoons  22  to a distal end of the other of the pontoons  22  to define the buoyant floater  12 . 
     It will be understood that when floating, a portion of the buoyant floater sub-assembly  60  and a portion of the buoyant floater  12  remain above the water line, as shown in  FIG.  7   . It will be further understood that the step of post-tensioning the buoyant floater sub-assembly  60  and the additional pontoons  22  to the buoyant floater sub-assembly  60  to define the buoyant floater  12  will occur above the waterline, and will therefore be in dry portions of the pontoons  22  and the central hub  24 . 
     The central column  26  may then be formed in sections on shore, such as described above, assembled on the first axial end wall  24 A of the central hub  24  while the buoyant floater  12  is floating, such as alongside a dock, and longitudinally post-tensioned. The braces  28  may then be attached between the pontoons  22  and the central column  26 , also while the buoyant floater  12  is floating alongside a dock. 
     The tower  14  and the wind turbine  16  may then be installed on the central column  26  while the buoyant floater  12  is floating, preferably above the submerged mass  36 , as described below. Alternatively, if the depth of the body of water BW permits it, ballast may be added to the buoyant floater  12  to move the buoyant floater  12  temporarily to the seabed S to install the tower  14  and the wind turbine  16 . 
     The depth of the mass  36  is variable, thus allowing for a shallow tow-out draft of about 10 m or less, that is comparable to a semi-submersible FOWT. Thus, the FOWT platform  10  has the mobility characteristics of a semi-submersible FOWT combined with the stability characteristics of a spar-type platform. 
     As configured in the embodiments described herein, the buoyant floater  12  has a tow-out or shallow draft of 7.53 m and the assembled FOWT platform  10  has an operational draft of 21.25 m. 
     A first embodiment of a method of assembling and deploying the mass  36  includes forming, such as by casting, sections (not shown) of the outer tank  40  in an on-shore location. Such sections may then be post-tensioned together. The assembled, but empty outer tank  40  may then be launched into the body of water BW by any desired method and allowed to float in the floating assembly area (not shown) wherein the buoyant floater  12  is located. 
     A second embodiment of a method of assembling and deploying the mass  36  includes forming the outer tank  40  as one large, monolithic section using conventional slip-forming or staged casting methods in an on-shore location. Once cast, the empty outer tank  40  may then be launched into the body of water BW by any desired method and allowed to float in the floating assembly area (not shown) wherein the buoyant floater  12  is located. 
     A first embodiment of the improved method of assembling and deploying the FOWT platform  10  into a body of water BW is shown schematically in  FIGS.  4 A through  4 F . 
     As shown in  FIG.  4 A , an empty outer tank  40  of the mass  36  may be launched from a dock or shoreline SL into the body of water BW by any desired method and allowed to float in the floating assembly area (not shown) wherein the buoyant floater  12  is located (the buoyant floater  12  is not shown in  FIGS.  4 A or  4 B , but is shown in  FIGS.  4 C through  4 F ). 
     The permanent ballast material  48  may then be added to the outer tank  40 , thus defining the mass  36 , and causing the mass  36  to sink to the seabed S, as shown in  FIG.  4 B . As the mass  36  sinks, the variable ballast space  50  fills with sea water. 
     The transit lines  56  and the suspension lines  38  may be preinstalled on the mass  36  and may be carried to the surface of the body of water BW with marker buoys attached to free ends of each of the transit lines  56  and the suspension lines  38 . 
     If not already launched into the body of water BW, the buoyant floater  12  may be launched and then floated, i.e., moved, to a position over the mass  36 . At this step of the method of assembling and deploying the FOWT platform  10  into a body of water BW, the ballast chambers in the buoyant floater  12  contain no ballast water, thus allowing the buoyant floater  12  to float. It is at this step also, that the tower  14  and the wind turbine  16  may then be installed on the central column  26  while the buoyant floater  12  is floating. 
     The temporary transit lines  56  and associated chain jacks  58  may now be used to lift the mass  36  to a point directly under the buoyant floater  12  to define a shallow draft configuration, i.e., the draft of 7.53 m, as shown in  FIGS.  4 C and  4 D . The installed suspension lines  38  are attached to the mass  36  and left slack. At this point, the FOWT platform  10  behaves as a rigid body characterized by a vertical center of gravity that is above the vertical center of buoyancy. Thus, FOWT platform  10  stability is achieved via a waterplane moment of inertia generated by the partially submerged pontoons  22  of the FOWT platform  10 . 
     The now assembled FOWT platform  10  may then be towed to an installation site in the body of water BW. At this point, the FOWT platform  10  continues to behave as a rigid body characterized by a vertical center of gravity that is above the vertical center of buoyancy. Thus, the FOWT platform&#39;s  10  natural periods may fall within a range of typical wave periods. Accordingly, this step of towing the assembled FOWT platform to the installation site in the body of water BW should only be conducted in relatively calm seas. 
     Alternatively, means other than the winches or chain jacks  58  may be used to raise and lower the mass  36  to and from the buoyant floater  12 . Alternative means for raising and lowering the mass  36  include, but are not limited to, a removable floatation device, such as an inflatable device, removable, external ballast tanks, and temporary barge support. Additionally, the mass  36  may be configured to float under or alongside the buoyant floater  12 . 
     Once the FOWT platform  10  has reached the installation site in the body of water BW, the mooring lines  18  attached to the pontoons  22  of the FOWT platform  10  may be connected to the anchors  20  in the seabed S. The mass  36  may then be lowered using the temporary transit lines  56  and associated chain jacks  58 , as shown in  FIG.  4 E . The temporary transit lines  56  may be removed from the mass  36  once the suspension lines  38  become taught, and thus carry the full weight of the suspended mass  36 , as shown in  FIG.  4 F . 
     The pontoons  22  may be fully submerged by flooding the internal ballast chambers with sea water. The FOWT platform  10  now rests at its design draft, as shown in  FIG.  4 E , and wherein the FOWT platform  10  is positioned in the body of water BW such that the waterline WL is at a mid-point of the central column  26 . 
     Although  FIGS.  1  through  4 F  illustrate the mass  36  having four fairleads  52  for the connection of suspension lines  38 , the mass  36  may include any desired number of fairleads  52 , configured for the attachment of any desired number of suspension lines  38 . For example, the mass  36  may include three fairleads  52 , two fairleads  52 , or a single fairlead  52 , as shown in  FIG.  6   , wherein an alternate embodiment of the mass  66  has only one fairlead  52  mounted to a first end wall  68  thereof. 
     Advantageously, by allowing the suspended mass  36  to be free to rotate and have its own natural period of vibration, it may be tuned to mitigate the motion experienced by the deployed buoyant floater  12 . This period of vibration may be obtained by precisely selecting the number of suspension lines  38 , their attachment point or points on the mass  36 , and tuning the center of gravity location and mass moment of inertia of the suspended mass  36 , to achieve a vibrational frequency that is favorable for reducing motion of the buoyant floater  12  and wind turbine nacelle  34 . 
     The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.