Abstract:
An apparatus for supporting an additional structure near a surface of a body of water, and a system which includes the apparatus and further includes the structure attached to the apparatus. The apparatus and the system are each configured to assume a rest position and orientation when the apparatus or system is floating at the surface and when the body of water is substantially still, where the rest orientation defines a vertical direction extending from the surface to a keel at a lowermost position of the apparatus. The apparatus includes a support member, which, in use, is attached to the additional structure; and buoyant units. Each buoyant unit is attached to the support member at or near the keel and extends from the keel in a longitudinal direction of the buoyant unit, which longitudinal direction defines an angle of approximately 35°-65° with respect to the vertical direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/982,258, filed Apr. 21, 2014, the disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a floatable support structure configured to be anchored to the sea floor and to support an additional device such as an offshore wind turbine. 
       BACKGROUND OF THE INVENTION 
       [0003]    A support structure for an offshore wind turbine must be stable and safe. Wind turbines can operate satisfactorily when motions due to wind current and waves are relatively small. However large motions of surge, sway, pitch, roll, yaw, and heave will result in inefficient operation or shutdown. Therefore, the support structure must have relatively small motions. The support structure motions are affected by the force of wind on the turbine blades, nacelle, and tower, by the wave and current forces on the submerged structure, by the period of the waves, by the period of the rotating turbine blades, and by the natural period of the entire structure, i.e. the time it takes to complete one cycle of motion when subjected to an excitation force. This period depends on the mass of the structure including the added mass of surrounding water, the stiffness of the structure itself (e.g. tower flexing and blade flexing), and the stiffness of the anchoring system of the structure. 
         [0004]    If the period of the support structure is synchronous with the period of the waves, the wave forces will be amplified, leading to disruption of operations and/or structural failure. Therefore it is desirable to ensure that the natural period of the support structure differs substantially from the wave periods for all reasonably expected waves. 
         [0005]    Vortex Induced Motions (VIMs) are another factor affecting the motions of a floating wind turbine support structure. VIMs are defined as motions caused by vortexes that form from current moving past a structure. 
       SUMMARY 
       [0006]    The disclosure relates to an apparatus for supporting an additional structure near a surface of a body of water. Also disclosed is a system which includes the apparatus and further includes the structure attached to the apparatus. The apparatus and the system are each configured to assume a rest position and orientation when the apparatus or system is floating at the surface and when the body of water is substantially still, where the rest orientation defines a vertical direction extending from the surface to a keel at a lowermost position of the apparatus. The apparatus includes a support member, which, in use, is attached to the additional structure; and buoyant units. Each buoyant unit is attached to the support member at or near the keel and extends from the keel in a longitudinal direction of the buoyant unit, which longitudinal direction defines an angle of approximately 35°-65° with respect to the vertical direction. 
         [0007]    Each of the buoyant units may have a certain cross-sectional area at a first position along the longitudinal direction, and a different cross-sectional area at a second position along the longitudinal direction. For example, each of the buoyant units may have a certain cross-sectional area at a first portion of the buoyant unit that is distal from the keel, and a smaller cross-sectional area at a second portion of the buoyant unit that is proximal to the keel. There may be a transitional portion of the buoyant unit between the first portion and the second portion, such as a tapered transitional portion. 
         [0008]    Alternatively, each of the buoyant units may have a cross-sectional area that is substantially constant along the longitudinal direction. 
         [0009]    The buoyant units may be symmetrically disposed around the vertical direction. 
         [0010]    The buoyant units may be three buoyant units, four buoyant units, or more than four buoyant units. 
         [0011]    At least a portion of each buoyant unit that is distal from the keel may include a buoyant material, whose density is lower than the density of the body of water. At least a portion of each buoyant unit that is proximal to the keel may include a ballast material, whose density is equal to or higher than the density of the body of water. 
         [0012]    The apparatus or system may further include at least one anchor line or tether, configured to attach the apparatus to a substantially stationary position within the body of water. The anchor line or tether may include one or more catenary anchor lines, each attached to one of the buoyant units. The anchor line or tether may include one or more tendons, each attached to one of the buoyant units, and configured to assume a substantially vertical orientation to anchor the apparatus or system to the floor of the body of water. 
         [0013]    The apparatus or system may further include a heave plate at the keel. 
         [0014]    The floatable structure may include a wind turbine and associated tower. The support member may be attached to the tower. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Exemplary embodiments will be described in more detail with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is an elevation view of an exemplary floatable support structure with a tower and turbine attached, and anchored to the sea floor with a vertical tendon at the keel and taut catenary anchors. 
           [0017]      FIG. 2A  is an enlarged elevation view of the support structure of  FIG. 1  without the tower and turbine attached, and with additional vertical tendons. 
           [0018]      FIG. 2B  is a view similar to  FIG. 2A , showing an alternative embodiment of the shape of buoyant units. 
           [0019]      FIG. 3  is a plan view of the support structure of  FIGS. 1 ,  2 A, and  2 B without the tower and turbine attached. 
           [0020]      FIGS. 4A-4D  are schematic illustrations showing the water plane area of an exemplary buoyant unit and its distance from the centroid of the structure, where: 
           [0021]      FIG. 4A  is a side view of a buoyant unit when the support structure is in its upright position; 
           [0022]      FIG. 4B  is a cross-sectional view showing the water plane area of the buoyant unit of  FIG. 4A  and the distance of the water plane area from the centroid of the structure; 
           [0023]      FIG. 4C  is a side view of the buoyant unit when the support structure is tilted significantly; and 
           [0024]      FIG. 4D  is a cross-sectional view showing the water plane area of the buoyant unit in the configuration of  FIG. 4C  and the distance of the water plane area from the centroid of the structure. 
           [0025]      FIG. 5  is an elevation view, similar to that of  FIG. 2 , showing a further embodiment of the support structure. 
           [0026]      FIG. 6  is a plan view of the support structure of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Exemplary embodiments described herein provide a floatable support structure configured to be anchored to the sea floor and to support an offshore wind turbine or other device. Some embodiments are considered particularly useful for deep waters, such as in water depths greater than about 40 meters, and some embodiments can be used in depths of over one thousand meters. 
         [0028]    Turning to  FIG. 1 , a floatable support structure  100  is shown supporting a wind turbine  200  and floating the wind turbine  200  on a water surface  302 . The floatable structure  100  is attached to the sea floor  304  by catenary anchor lines  402  or vertical tendons  404  or, as illustrated, by both catenary anchor lines  402  and a vertical tendon  404 . 
         [0029]    With further reference to  FIGS. 2A ,  2 B, and  3 , the structure  100  includes buoyant units  102  which meet at the keel  104  of the structure and extend slopingly upward from the keel  104 . (As used herein, the keel  104  should be understood to be the bottommost point or area of the structure  100  when the structure  100  is in its upright position, which position is seen in  FIGS. 1 ,  2 A, and  2 B. In other words, the horizontal line at the bottom of the structure  100  near where the tendon  404  attaches to the structure  100  in  FIGS. 1 ,  2 A, and  2 B is the keel.) The structure  100  also includes a generally central support member  106  which attaches to the buoyant units  102  at the keel  104  and extends generally vertically therefrom. The support member  106  is configured to attach to and to support a tower  202  of the turbine  200 . The buoyant units  102  and the support member  106  are connected to one another by various braces  108 ,  110  and connecting members  112 . 
         [0030]    The buoyant units  102  are partly submerged in use, i.e. partly below the water surface  302 , as shown. The illustrated embodiments include three such buoyant units  102 , which slope outward from at or near the keel  104  to above the water surface  302 . In some presently preferred embodiments, three or more buoyant units  102  are provided. Each buoyant unit  102  is a generally tubular member whose diameter may vary along its length, as will be discussed in detail later. Each buoyant unit  102  intersects the support member  106  at an angle θ of approximately 35-65°. In other words, the longitudinal direction of each buoyant unit  102  is about 35-65° from the longitudinal direction of the support member  106 , which is vertical when the structure is in its upright position, i.e. not being rocked by waves or wind. 
         [0031]    In the embodiment illustrated in  FIG. 2A , each buoyant unit  102  is a generally tubular member whose diameter varies along its length to provide a larger cross-sectional area at the top portion of the buoyant unit  102  than at the bottom. In the alternative embodiment illustrated in  FIG. 2B , the diameter is substantially constant along the length. Each buoyant unit  102  includes several compartments  102   a ,  102   b ,  102   c , separated by water-tight bulkheads  114 , along its length. There may also be smaller sub-compartments (not illustrated) within these compartments  102   a ,  102   b ,  102   c . The upper compartments  102   a  may contain buoyant material, and the lower compartments  102   c  may contain ballast material. The diameter and length of the upper compartment  102   a  of each buoyant unit can be selected to achieve the required buoyant force and water plane area and resulting moment of inertia of the structure  100 &#39;s cross section at the waterline  302 . An intermediate, or “transition” section  102   b  connects the upper part  102   a  of each buoyant unit  102  to the lower part  102   c  of the buoyant unit  102 . In the embodiment illustrated in  FIG. 2A , the lower part  102   c  has a smaller cross-sectional area than the upper part  102   a . Depending on requirements for stability, a large portion of each buoyant unit  102  will be filled with ballast. Therefore ballast requirements as well as buoyancy requirements will dictate the length and cross-sectional area of each part  102   a ,  102   b ,  102   c  of each buoyant unit  102 . 
         [0032]    In a presently preferred embodiment, both the buoyant units  102  and the support member  106  are steel tubular members, which may include ring stiffeners and/or longitudinal stiffeners on the interior to hold the round shape and to resist hydrostatic pressure. In a presently particularly preferred embodiment, the units  102  and member  106  include ring stiffeners, and may additionally include longitudinal stiffeners. 
         [0033]    The tower  202  is supported by the support member  106 . The tower  202  extends from the support member  106  to high above the water surface  302  to support the wind turbine  200 . In more detail, the tower  202  supports the nacelle  204  which supports the turbine rotor blades  206 , and which contains the controls, the generator, and other required components. 
         [0034]    The buoyant units  102  are connected to the support member  106  by braces  108 . The buoyant units  102  are connected to one another by braces  110 . Near the keel  104 , the buoyant units  102  are further connected to the support member  106  by connecting members  112 . 
         [0035]    The support structure  100  is mounted to the sea floor  304  by catenary anchor lines  402 , which may include, for example, chain and/or wire rope, and/or polyester. 
         [0036]    The support structure  100  may additionally or alternatively be attached to the sea floor  304  by a vertical tether such as a tendon or tendons  404 . In some embodiments, the tether or tethers may be wire ropes, chains, polyester ropes, or steel or metal tubes. By selecting an appropriate length, modulus of elasticity, and/or cross-sectional area of the tendon or tendons (in addition to further design considerations which will be discussed later with reference to  FIGS. 4A-4D ), the natural period of the structure  100  in heave, surge, sway, pitch, roll, and yaw can be made not to coincide with the wave period of any waves that can be expected in the particular area at which the support structure  100  is installed. As is further seen in  FIGS. 2A and 2B , the tendon or tendons  404  can be attached to the structure  100  at or near the keel  104  and/or at one or more of the buoyant units  102 . In a presently preferred embodiment, one tendon  404  is attached at each of the buoyant units  102 . In practice, it is unlikely that three anchor lines  402  would be used in conjunction with three tendons  404  at the buoyant units  102  and an additional tendon  404  at the keel  104 , as is illustrated. These illustrations are provided for the sake of completeness, to illustrate many possible attachment locations of anchor lines and tendons. 
         [0037]    The tendon or tendons  404  may be attached to the sea floor  304  via an anchor or anchors  406 . Depending on the applied force from the tendon  404  and the sea floor conditions, each anchor may be a suction pile, a driven pile, a drilled and grouted pile, a gravity anchor, or any anchor with the capacity to transmit the tendon forces to the sea floor. 
         [0038]    It will be appreciated that the support structure  100  can be fabricated, assembled, and launched at a quay, uprighted, then outfitted with the tower  202  and wind turbine  200  all at quay site. The assembled wind turbine  200  and support structure  100  can then be towed in upright position to its installation site and then be attached to previously installed anchors  406  or catenary anchor lines  402 . 
         [0039]    In the illustrated embodiments, the support structure  100  is shown supporting an offshore wind turbine  200  and associated tower  202 . The structure  100  can also be used to support any other device, such as an electrical substation, or for oil and gas exploration and production. 
         [0040]    In some unillustrated embodiments, the support structure  100  is used to support a platform for a support station rather than a turbine  200 . Since these platforms are generally rectangular in plan view, the structure will typically have four buoyant units  102  rather than the three buoyant units  102  illustrated, each of which will attach to the platform at or near a corner thereof. In a presently preferred embodiment, the support member  106  extends through a blind hole or a through hole at the center of the platform to provide further support. 
         [0041]    Turning now to  FIGS. 4A-4D , the stability of the exemplary structure  100  will now be discussed. It will be appreciated that the stability of the structure  100  is proportional to I/V, where I is the moment of inertia of the water plane area (for the hydrodynamic definition of moment of inertia, as opposed to the mass moment of inertia), and V is the volume of water displaced by the structure  100 . For the sake of simplicity,  FIGS. 4A-4D  illustrate the stability of a single one of the buoyant units  102 . It will further be appreciated that the moment of inertia I is proportional to A           2 , where A is the water plane area of the buoyant unit  102 , in other words, the cross-sectional area of the buoyant unit  102  that is presented to the surface of the water (i.e. the cross-section taken vertically, as opposed to along the longitudinal axis of the buoyant unit  102 ), and           is the horizontal distance from the centroid of the water plane area of the buoyant unit  102  to the centroid of the water plane area of the entire structure  100 . 
         [0042]    Turning to  FIG. 4A , one of the buoyant units  102  is shown along with the support member  106 , when the structure  100  is upright, i.e. not being rocked by waves or wind. The additional buoyant units  102  are omitted from these FIGS. for simplicity. The orientation of the buoyant unit  102  in  FIG. 4A  is the same as that illustrated in  FIGS. 1 ,  2 A, and  2 B. Assuming a circular cross-section for the upper part  102   a  of the buoyant unit  102  (taken along the longitudinal direction of the buoyant unit  102 ), the water plane area, i.e. the cross-section presented to the water surface  302 , is elliptical, as seen in  FIG. 4B . The minor diameter d minor  of the ellipse is equal to the cross-sectional (along the longitudinal axis) diameter of the unit  102 , d minor =d unit . The major diameter d major,1  is the distance between where the left and right edges of the unit  102  intersect the waterline  302  in  FIG. 4A . Elementary trigonometry tells us that this distance d major,1 =d unit /sin α, where α is the angle at which the unit  102  intersects the water line  302 , i.e. α=90°−θ. The horizontal distance            1  from the center of the ellipse to the centroid of the structure  100 , which in this case is the center of the water plane area of the support unit  106 , is also shown in  FIG. 4B . 
         [0043]    Turning now to  FIG. 4C , the structure  100  is shown tilted dramatically to the left. Again, the water plane area of the buoyant unit  102  is elliptical, as seen in  FIG. 4D . The minor diameter d minor  is again equal to d unit . However, d major,2  now much larger. d major,2 =d unit /sin β, where β is the new angle at which the unit  102  intersects the water line  302 . The horizontal distance            2  from the centroid of the water plane area of the buoyant unit  102  to the centroid of the structure  100  is also larger than that of  FIG. 4B , since the center of the water plane area of the buoyant unit  102  is farther to the left due to the larger major diameter d major,2 . Note that in this case, the centroid of the water plane area of the entire structure  100  is no longer coincident with the centroid of the water plane area of the support member  106 , but is shifted slightly to the left. However, in practice,            2&gt;             1 . Thus, the contribution of each buoyant unit  102  to the moment of inertia I of the structure  100 , which is proportional to A           2 , increases when the respective unit  102  tips, providing increased stability when the structure  100  is impacted by waves or wind. 
         [0044]    As was mentioned in the background section, a floating support structure should be subjected to relatively small motions when affected by wind, waves, and current in the directions of surge, sway, pitch, roll, yaw, and heave. The natural period depends on, among other parameters, the mass of the structure. The sloping buoyant units  102  of the above-described exemplary embodiments have a plan view that presents a large area exposed to water above and below the units. This results in a large added mass when considering heave, i.e. vertical motions, of the floatable structure. 
         [0045]    Turning now to  FIGS. 5 and 6 , in a further alternative embodiment, the structure  100  further includes a heave plate  150 . It will be appreciated that, in embodiments with a vertical tendon or tendons  404 , heave may not be much of a concern once the structure  100  is anchored to the sea floor  304  with the tendon or tendons  404 , but heave can be a problem when the structure  100  is being towed from the quay to the installation site. Furthermore, in embodiments without a vertical tendon or tendons  404 , such as embodiments in which the structure  100  is secured with catenary anchor lines  402  and not tendons  404 , heave may be a concern even once the structure  100  is installed. Therefore, in some embodiments, the structure  100  further includes the heave plate  150 , which minimizes heave while the structure  100  is being towed to the installation site and/or after installation. 
         [0046]    A heave plate is typically generally plate-shaped; i.e. has two roughly parallel planar surfaces and is relatively thin in the direction perpendicular to these planes. The planar surfaces are typically disposed perpendicular to the direction of heave (i.e. roughly horizontal, parallel to the surface of the water  302  and to the sea floor  304 ) for increasing the effective mass of the structure to which they are attached (in this case the structure  100 ). A plate so attached affects the dynamic behavior of the structure  100  by increasing the effective mass and the viscous drag in the heave (vertical) direction. The heave plate  150  can be any relatively thin square, circular, rectangular, or any other shape of plate, and can either be solid or have holes punched in it. 
         [0047]    In the illustrated embodiment, the heave plate  150  is circular, and is strengthened by hexagonal ridges  152 . The shape of the plate  150  and ridges  152  are exemplary only and are in no way intended to be limiting. The heave plate  150  may have any shape in plan view and may have any number and configuration of ridges and/or holes on or in it. 
         [0048]    To ensure that the period of the support structure differs substantially from all expected wave periods, the stiffness of the lines  402 ,  404  can be selected accordingly, for example, by selecting an appropriate material. Additionally or alternatively, the mass of the structure can be selected, for example, by adding or subtracting ballast. 
         [0049]    Since VIMs are particularly pronounced on current moving past vertical, column-like elements, in presently preferred embodiments, the buoyant units are sloped. The result is that the formation of vortexes are disrupted and VIMs are diminished. 
         [0050]    Another feature of some of the above-described exemplary embodiments is that the very large cost of offshore construction can mostly be avoided. Except for the preset anchors and their attachment to the floatable structure, the fabrication, assembly, launching and outfitting of the complete wind turbine structure can be accomplished at or near the quay. This will, however, require reasonably deep water at the quay and no overhead obstructions that would interfere with towing to the installation site. This feature is aided by the sloping buoyant units and the fact that they intersect the water surface with a large elliptical waterplane area at a large distance from the centerline of the structure. A requirement for stability for a floating body is that the body&#39;s metacenter be above the center of gravity of the body. The metacentric height (i.e. the distance between the center of gravity and the metacenter) is equal to the moment of inertia of the waterplane area divided by the volume of displaced water plus or minus the distance between the centers of buoyancy and gravity when the body is in equilibrium. The configuration of the embodiments described herein, with a large separation between each buoyant unit&#39;s waterplane area, results in a high metacenter and good stability both when free floating while being towed and also when anchored. 
         [0051]    The combination of small motions (due to the structure having a natural period that is not synchronous with wave periods), reduced vortex induced motions, and good stability due to a high metacenter, results in a safe and stable floating structure. The fact that most offshore construction can be avoided results in an economical installation. 
         [0052]    As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. Many other embodiments are possible without departing from the essential characteristics thereof. Many other embodiments are possible without deviating from the spirit and scope of the invention. These other embodiments are intended to be included within the scope of the present invention, which is set forth in the following claims.