Patent Publication Number: US-2022219791-A1

Title: Mooring structure for ocean wave energy converters

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent document is related to co-filed U.S. patent application Ser. No. 17/149,272, entitled “Mooring Latch for Marine Structures,” which is hereby incorporated by reference in its entirety. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under grant No. DE-EE0008626 awarded by the Department of Energy. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     A Wave Energy Converter (WEC) is a system for converting the energy from waves into other forms of energy. Particularly, WECs may be deployed in the ocean to produce electrical energy from ocean waves. WECs are a desirable energy source because WECs can produce electrical power without producing green house gases such as CO 2  (or any waste product) and can produce electrical energy when other “green” energy systems are dormant or inefficient. For example, WECs can efficiently produce power at night when solar energy systems are inoperable and produce power in low wind conditions where wind power is inefficient. 
     A Cyclic or Cycloidal Wave Energy Convert (CycWEC) is a type of WEC that uses hydrofoils that interact with incoming ocean waves to create lift that applies torque to rotate a shaft and drive a conventional electrical generator. The hydrofoils in a CycWEC are mounted on standoffs or other lever arm structures that connect the hydrofoils to the driven shaft and that cause the hydrofoils to move in a circle around the driven shaft and trace a cycloidal path relative to the traveling waves. U.S. Pat. No. 7,686,583, entitled “Cyclical wave energy converter” and U.S. Pat. No. 7,762,776, entitled “Vortex Shedding Cyclical Propeller,” which are hereby incorporated by reference in their entirety, further describe examples of the structure and operation of CycWECs. CycWECs have been shown to provide high energy conversion efficiency when compared, for example, to WEC systems with floats that move up and down and use additional mechanical systems, e.g., a crankshaft system, to generate torque to turn a generator. 
     Any OWEC that extracts power from incoming ocean waves experiences reactive forces as a result of the power extraction according to the Newton&#39;s laws of motion. A mooring system may apply a counteracting force to the OWEC to maintain the OWEC&#39;s position for operation. Due to the rotating particle motions in wave-induced flow fields, the active forces on an OWEC may orient in any direction relative to the wave crest, which makes traditional mooring lines inadequate for OWECs since traditional mooring lines only transfer loads in one direction (that of the line) and only in tension. A mooring system for a OWEC generally needs to be able to survive variable ocean conditions and continue to position the OWEC for efficient wave energy conversion. A mooring system for a CycWEC may, for example, need to position the CycWEC at a desired ocean depth that may be optimized depending on wave amplitude and may need to regularly reorient the CycWEC depending on the direction of wave propagation. Further, mooring systems ideally should be cost effective to manufacture, install, operate, and maintain. 
     SUMMARY 
     In accordance with an aspect of the invention, a rigid mooring system attaches an Ocean Wave Energy Converters (OWEC) to the ocean floor using multiple extendible members, e.g., extendible legs and braces, that can be actively controlled for installation, operation, maintenance, and decommissioning of the OWEC. In particular, changes in the lengths of the extendible members can maneuver, position, and orient, e.g., adjust height, yaw, and roll of, an OWEC to support operation of a variety of types of OWEC in a variety of ocean conditions. The extensible system may further facilitate installation of the mooring system and OWEC, may lift the OWEC out of the water or to depth suitable for maintenance, or submerge the OWEC to an ocean depth that protects the OWEC (and the mooring system) from damage by storms or other surface conditions or events. 
     Some examples of the present disclosure are particularly useful for OWECs that have a large aspect ratio, e.g., OWECs commonly referred to in the industry as wave attenuators or wave terminators, because the mooring system can adjust the orientation of an OWEC relative to changing wave patterns. For wave energy converters that need to be aligned to an incoming wave direction, a mooring system according to an example of the present disclosure allows for adjusting the orientation of the OWEC. 
     Some examples of the mooring system can fully support a range of activities encountered during the life cycle of a wave energy converter, from commissioning to operation, storm survival and maintenance throughout the operational life of a converter, and all the way to the decommissioning of the system. Examples of the mooring systems disclosed herein do not require external stabilized platforms, dynamic positioning vessels, heavy lifting equipment, divers, or Remotely Operated Vehicles (ROVs) for installation, operation, or maintenance. The mooring systems can provide a stabilized platform with adjustment of the wave energy converter&#39;s submergence as well as orientation relative to incoming waves. For maintenance, the mooring system in some examples of the invention can lift the platform entirely out of the water to allow access to all components of the system that may require scheduled or unscheduled maintenance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-1, 1-2, 1-3, and 1-4  respectively show perspective, top, front, and end views of a Cycloidal Wave Energy Converter (CycWEC) with a four-legged mooring system in an operational configuration. 
         FIG. 2  shows a jacking or actuation system that may be incorporated in an extensible member such as a leg or brace in accordance with an example of the present disclosure. 
         FIG. 3  shows a mooring platform in accordance with an example of the present disclosure including joints such as Cardan connection points for extendible members and mooring connectors. 
         FIG. 4-1  shows an example of a CycWEC with a mooring platform in a stowed leg configuration. 
         FIG. 4-2  shows a CycWEC with a mooring platform having three legs unfolded and one leg in the stowed position. 
         FIG. 4-3  shows a CycWEC with a mooring platform with one leg unmoored and lifted to the surface for maintenance. 
         FIGS. 5-1  show a CycWEC with a mooring platform in a yawed position. 
         FIG. 5-2  shows a CycWEC with a mooring platform with legs fully closed for a protected or survival position. 
         FIG. 5-3  shows a CycWEC with a mooring platform with legs fully extended for a maintenance position. 
         FIG. 6  shows a top view of a line cluster of moored CycWECs at a non-zero wave angle and having shared mooring points. 
         FIG. 7  shows a perspective view of a six-legged mooring system in accordance with an example of the present disclosure. 
         FIG. 8  shows a perspective view of a five-legged mooring system in accordance with an example of the present disclosure. 
         FIG. 9  is a block diagram illustrating a control system for a wave energy converter and mooring system in accordance with an example of the present invention. 
     
    
    
     The drawings illustrate examples for the purpose of explanation and are not of the invention itself. Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION 
     A mooring system for an ocean wave energy converter (OWEC) transfers loads from the OWEC structure through a platform with at least three extensible legs attached to mooring points that attach to the ocean floor. Each leg and other extensible, structural members in the mooring system may have a jacking or actuation system to extend or shorten the member. Joints attach the legs to the platform and mooring points and provide two degrees of freedom for rotations about where the leg attaches to the platform or to a mooring point. Neither three nor four legs with such joints are sufficient to restrict all six degrees of freedom, e.g., three degrees of freedom in translation in X, Y, and Z directions and three degrees of freedom in rotation in XY, YZ and ZX planes, of the platform, so additional extensible braces or legs (also having attachment joints providing two degrees of rotational freedom) may be added to stabilize the platform. The platform with a sufficient number of legs and/or braces is stable/rigid when lengths of the legs or braces are fixed, but the mooring system permits changes to the pose or configuration of the platform through adjustments or changes of the lengths of one or more of the legs or braces. 
       FIGS. 1-1, 1-2, 1-3, and 1-4  respectively show a perspective view, a top view, a front view, and a side view of an OWEC system  100  in accordance with an example of the present disclosure. In the illustrated example, OWEC system  100  includes a OWEC  110  that is on or forms part of a platform  120  of a mooring system  130  having eight structural members, four legs  131 ,  132 ,  133 , and  134  and four braces  121 ,  122 ,  123 , and  124  that are extensible. Structural members  131 ,  132 ,  133 ,  134 ,  121 ,  122 ,  123 , and  124  are extensible in the sense that their lengths may be changed, e.g., extended or contracted, while the configuration of mooring system  130  is being changed, but the lengths of structural members  131 ,  132 ,  133 ,  134 ,  121 ,  122 ,  123 , and  124  may be rigidly fixed while the configuration of mooring system  130  is being maintained. Structural members  131 ,  132 ,  133 , and  134  and  121 ,  122 ,  123 , and  124  are sometimes distinguished herein as being legs or braces depending on whether the structural member has an end that attaches to an external structure, e.g., to a mooring point anchored to the ocean floor, or attaches internally to other elements of mooring system  130 . While  FIGS. 1-1, 1-2, 1-3, and 1-4  show the example of OWEC  110  being a CycWEC, more generally, other types of OWEC may be similarly moored. 
     CycWEC  110  as shown in  FIG. 1-1  includes one or more electrical generators  112  that may be mounted on platform  120 . Each generator  112  may be any type of electrical generator, e.g., a dynamo or an alternator, that makes use of electromagnetic induction or other processes to transform mechanical rotation into direct or alternating electrical current. A shaft  114  of CycWEC  110  is coupled to generators  112 , e.g., directly or through a transmission, so that rotation of shaft  114  causes generators  112  to generate electrical energy. In the illustrated configuration, shaft  114  extends between two generators  112  mounted on platform  120  and is free to rotate relative to platform  120 . One or more hydrofoils  116  attaches to shaft  114  through respective pairs of spoke or standoff mechanisms  118  near the ends of hydrofoils  116  and shaft  114 , and in the example of  FIG. 1-1 , CycWEC  110  includes two hydrofoils  116  positioned 180° apart relative to rotation of shaft  114 . In operation, a wave passing through CycWEC  110  in a direction other than parallel to the lengths of hydrofoils  116  (and for best efficiency in a direction perpendicular to the lengths of hydrofoils  116 ) interacts with hydrofoils  116  causing lift. The wave-induced lift and the moment arm of standoff mechanisms  118  create torque to turn shaft  114 , causing generators  112  to produce electrical energy. For efficient production of lift and the resulting torque, a control system (not shown) for CycWEC  110  may operate standoff mechanisms  118  control the attack angles of hydrofoils  116  as hydrofoils  116  rotate and each wave passes. In particular, the attack angle may vary according to a pitching schedule that depends on the amplitude, frequency, and phase of the interacting waves and/or the rotational angle of the hydrofoil  116  about shaft  114 . See U.S. Pat. No. 7,762,776, entitled “Vortex Shedding Cyclical Propeller,” which describes example uses of pitching schedules. 
     Mooring system  130  uses joints  141 ,  142 ,  143 , and  144 , e.g., universal or Cardan joints, to attach the bottoms of legs  131 ,  132 ,  133 , and  134  to mooring points  151 ,  152 ,  153 , and  154  and similar joints  146 ,  147 ,  148 , and  149  to attached respective legs  131 ,  132 ,  133 , and  134  to platform  120 . Each joints  141 ,  142 ,  143 ,  144 ,  146 ,  147 ,  148 , or  149  is rigid with regard to compression or tension but permits rotation of the attached leg  131 ,  132 ,  133 , or  134  about two axes through the joint. Mooring points  151 ,  152 ,  153 , and  154  may be fixed mounting structures directly anchored in or on the ocean floor or fixed on a platform (not shown) that extends to and anchors on the ocean floor. Each joint  146 ,  147 ,  148 , or  149  connecting a leg  131 ,  132 ,  133 , or  134  to platform  120  is such that no bending moments are transferred from the leg  131 ,  132 ,  133 , or  134  to the platform  120 , and each leg  131 ,  132 ,  133 , or  134  is free to rotate around a horizontal and vertical axis freely while being firmly connected to platform  120  to transmit tension and compression loads between legs  131 ,  132 ,  133 , and  134  and platform  120 . A common joint mechanism to achieve this is a Cardan joint, universal joint, or U-joint, but other connections such as swivel ball joints may be used, as desired, to achieve this function. 
     The four extensible braces  121 ,  122 ,  123 , and  124  connect between platform  120  and respective legs  131 ,  132 ,  133 , and  134  using similar joints, e.g., U-joints or Cardan joints. For fixed lengths of extensible legs  131 ,  132 ,  133 , and  134  and braces  121 ,  122 ,  123 , and  124 , legs  131 ,  132 ,  133 , and  134  and braces  121 ,  122 ,  123 , and  124  together restrict or constrain all six degrees of freedom of motion of platform  120  and mounted CycWEC  110 , with redundancy of two degrees of freedom. A four-leg configuration with four braces is useful in order to maintain a fully constrained platform for a fail-safe feature in the event of partial failures or to remain fully constrained when unloading some of the legs or braces for maintenance. However, a three-leg configuration with three braces or equivalently a six-leg configuration with no braces could alternatively be employed for a fully constrained structure having fewer elements. Since OWEC  110  may create large loads, a particular advantage of mooring system  130  is that the legs and braces are all loaded in tension/compression only and not in bending which reduces structural costs substantially because smaller diameter structural members with thinner wall material can be used. 
     Legs  131 ,  132 ,  133 , and  134  are not all horizontal or vertical and generally converge from the wider separations X and Y of mooring points  151 ,  152 ,  153 , and  154  to the more closely space attachments of legs  131 ,  132 ,  133 , and  134  to platform  120 . In the configuration of  FIGS. 1-1 to 1-4 , mooring points  151 ,  152 ,  153 , and  154  are at the corners of a horizontal rectangle, but more generally, mooring points  151 ,  152 ,  153 , and  154  are not required to be coplanar or in a rectangular formation. 
     The lower end of each leg  131 ,  132 ,  133 , and  134  may attach to mooring points  151 ,  152 ,  153 , and  154  and the ocean floor in a semi-permanent fashion by means of a mooring latch. The function of a mooring latch is to independently attach and release each leg  131 ,  132 ,  133 , or  134  to and from respective mooring point  151 ,  152 ,  153 , or  154 . A variety of latch systems may be used to attach the legs to the mooring points, and co-file U.S. Pat. App. entitled “Mooring Latch For Marine Structures,” which is hereby incorporated by reference above, particularly describes a mooring latch suitable for connecting legs  131 ,  132 ,  133 , and  134  to mooring points  151 ,  152 ,  153 , and  154 . Each mooring latch may incorporate or provide a free-to-rotate joint  141 ,  142 ,  143 , or  144  but is fixed in tension and compression kinematics the same as the joint  146 ,  147 ,  148 , or  149  at the other end of the leg  131 ,  132 ,  133 , or  134 . In addition to being connected at both ends, each leg  131 ,  132 ,  133 , or  134 , along its length, features a third connection point or joint to which the associated brace  121 ,  122 ,  123 , or  124  attaches. 
     Legs  131 ,  132 ,  133 , and  134  and braces  121 ,  122 ,  123 , and  124  are independently adjustable in length. A jacking or actuation system of any type suitable for the offshore operation may be employed to adjust the length of each leg  131 ,  132 ,  133 , or  134  or brace  121 ,  122 ,  123 , or  124 . For example, the jacking system may be internal or external to the legs  131 ,  132 ,  133 , and  134  and braces  121 ,  122 ,  123 , and  124  and may employ either a pin and hydraulic cylinder or a rack and pinion system that are known for use in jack-up platforms. The illustrated system  100  includes legs  131 ,  132 ,  133 , or  134  and braces  121 ,  122 ,  123 , and  124  having holes for a pin and hydraulic cylinder jacking system. 
       FIG. 2  illustrates a portion of an extensible, structural member including an example of a jacking or actuation system  200  that may be incorporated in any of the extensible structural members. A structural member may particularly include telescoping tubes  210  and  220 , e.g., where tube  210  may slide into tube  220 . Tube  210  includes one or more linear gears or racks  212  that engage a pinion or gear in a drive system  222  mounted on tube  220 . Electrical motors  224  can be operated to turn or drive the pinions in drive systems  222  that engage racks  212  to pull tube  210  further into tube  220  and shorten the length of the extensible member or to push tube  210  further out of tube  220  to increase the length of the extensible member. Jacking system  200  and particularly drive system  222  may further include a brake to rigidly hold or maintain the length of the extensible member when a desired length has been reached or a clutch to allow tube  210  to freely slide relative to tube  220  while other jacking systems alter the length or lengths of other structural members. Jacking systems  200 , when incorporated in each leg  131 ,  132 ,  133 , and  134  and brace  121 ,  122 ,  123 , and  124  may make the length of each leg and brace independently (or dependently) adjustable, which may be used to reconfigure mooring system  130  for various purposes during the installation, operation, maintenance, or decommissioning of OWEC system  100 , as described further below. 
     The end view of  FIG. 1-4  illustrates a configuration of OWEC system  100  that keeps platform  120  in a position where CycWEC  110  is fully submerged and at a suitable depth D below a surface level  160  of the ocean. Depth D may be selected, e.g., adjusted, based on the amplitude of incoming waves and the diameter of the circular path of hydrofoils  116  in CycWEC  110 , so that CycWEC  110  can efficiently interact with surface waves  162  with no interference from platform  120  that might otherwise reduce the power that CycWEC  110  could extract from the wave. In general, a vertical distance Z of a CycWEC  110  relative to fixed mooring points  151  to  154  (or the ocean floor or other fixed location) depends on lengths of legs  131 ,  132 ,  133 , and  134 . In  FIG. 1-4 , simultaneously increasing or decreasing lengths of legs  131  and  132  increases or decreases vertical distance Z. A control system (described further below) may control jacking systems in mooring system  130  to actively alter the lengths of legs  131 ,  132 ,  133 , and  134  and thereby change vertical distance Z to provide a desired depth D in changing ocean conditions, e.g., changing tides or wave amplitudes. As also described further below, the end of platform  120  shown in  FIG. 1-4  may be shifted horizontally through control of changes in length of leg  131  relative to the length of leg  132 . While changes in the lengths of legs  131  and  132  may also move the end of platform  120  opposite from the end illustrated in  FIG. 1-4 , the effect on the position of the opposite end of platform  120  may be negligible. 
       FIG. 3  shows an example configuration of platform  120  without legs  131 ,  132 ,  133 , and  134  or braces  121 ,  122 ,  123 , and  124  attached. Platform  120  may be constructed using welded rectangular or cylindrical tubular structures to form a truss or beam  314 . OWEC mounting structures  312 , brace attachment points  321 ,  322 ,  323 , and  324 , and leg attachment points  331 ,  332 ,  333 , and  334  attach to or extend from truss or beam  314 . Additional leg mounting points  341 ,  342 ,  343 , and  344  on platform  120  provide temporary attachment points for the bottom ends of legs  131 ,  132 ,  133 , and  134  when the mooring system is in a stowed configuration, which may be used for transportation of the OWEC system as described further below. Platform  120  also has “feet”  351 ,  352 ,  353 , and  354  on which platform  120  can rest during assembly of the OWEC, legs, and braces on platform  120  before the OWEC system is moved into the ocean. Truss or beam  314  may be sized to extend the length of the OWEC to be mounted on platform  120 , and OWEC mounting structures  312  extend generally upward from the ends of truss or beam  314  to a height that depends on lateral dimensions of the OWEC and the spacing needed to prevent platform  120  from interfering with wave energy conversion when the OWEC is at operational depths. Further dimensions of OWEC mounting structures  312  and beam  314  in a particular platform  120  may take any configuration needed to connect or mount a desired OWEC to the legs and braces and provide necessary stability to oppose expected reaction forces on the OWEC. For a CycWEC such as shown in  FIG. 1-1  as an example, mounting structure  312  may extend to a height that accommodates the cycloidal movement of hydrofoils  116 , e.g., about 10 m to 20 m high for a OWEC radius of 6 m, and beam  314  may have a length that accommodates the lengths of hydrofoils  116 , e.g., about 70 m to 90 m for a hydrofoil with a span of about 60 m. The width of mounting structure  312  may be similar to the height of mounting structure  312  and must be sufficient to withstand expected reactive torques and forces on the OWEC and platform  120 . 
     In the example of  FIG. 3 , attachment points  331 ,  332 ,  333 , and  334  for legs  131 ,  132 ,  133 , and  134  are symmetrically located at or near four corners of platform  120 . The range of extension in the lengths of legs  131 ,  132 ,  133 , and  134  in general depends on the water depth at the installation location of the mooring system. The maximum length of a leg  131 ,  132 ,  133 , and  134  may be selected based on the requirement to lift the OWEC out of the water, and the minimum length of a leg  131 ,  132 ,  133 , and  134  may be selected based on the desired depth needed for storm survival at the installation location. 
     Braces  121 ,  122 ,  123 , and  124  attach to respective legs  131 ,  132 ,  133 , and  134  at one end and attach to attachment points  321 ,  322 ,  323 , and  324  on platform  120  with their other ends. The locations of brace attachment points  321 ,  322 ,  323 , and  324  on platform  120  differ. Referring to  FIG. 1-1  and  FIG. 3 , a pair of legs  132  and  133  have their braces  122  and  123  attached to attachment points  322  and  323  on the short sides or ends of platform  120 . This orients braces  122  and  123  to the counteract shaft torque generated by the mounted OWEC. Braces  122  and  123  are sometimes referred to herein as torque braces  122  and  123  since braces  122  and  123  prevent rotation of the platform  120  around the wave energy converter shaft  114 . Conversely, a pair of braces  121  and  124  prevents lateral swaying motion of platform  120  along the direction of shaft  114  and are sometimes referred to herein as lateral braces  121  and  124 . Attachment points  321  and  324  of braces  121  and  124  to platform  120  are thus located along the long side, e.g., beam or truss  314 , of platform  120  as shown in  FIG. 3 . 
     The joints of the mooring system allow reconfiguration of the mooring system throughout the life cycle of an OWEC. In particular, mooring systems as described herein may be reconfigured by changing the lengths (and, as a result of the length change, the connection angles) of the extensible structural members during different operating stages of an OWEC. 
       FIGS. 4-1, 4-2, and 4-3  show configurations of system  100  that may be used for installation, maintenance, decommissioning of system  100 .  FIG. 4-1  particularly shows a stowed leg configuration  410  of a mooring system  130  on which an OWEC  110  is mounted. Stowed leg configuration  410  may be used during commissioning and decommissioning an OWEC as well as storage and shipping of a complete OWEC system including both OWEC  110  and mooring system  130 .  FIG. 4-1  shows all four legs  131 ,  132 ,  133 , and  134  disconnected from mooring points  151 ,  152 ,  153 , and  154 , rotated about joints  146 ,  147 ,  148 , and  149  into a horizontal plane, and arranged along the long side of platform  120 . The free ends (or bottoms) of legs  131 ,  132 ,  133 , and  134  may respectively engage mating features  341 ,  342 ,  343 , and  344  on platform  120  as shown in  FIG. 3 . Mating features  341 ,  342 ,  343 , and  344  may have the same shape as mooring points  151 ,  152 ,  153 , and  154  so that the same mooring latches on the ends of legs  131 ,  132 ,  133 , and  134  may mate with and securely latch onto either mooring points  151 ,  152 ,  153 , and  154  or mating features  341 ,  342 ,  343 , and  344  of platform  120 . For stowed configuration  410 , legs  131 ,  132 ,  133 , and  134  have lengths adjusted to fit in the stowed positions, and legs  131 ,  132 ,  133 , and  134  and platform  120  may be sealed and fully or partly filled with air to provide flotation when positioning the system in the ocean. The buoyancy of the entire system may be controlled such that the buoyancy of legs  131 ,  132 ,  133 , and  134  plus the buoyancy of platform  120  support wave OWEC  110  in a position clearly over the water, e.g., fully out of the water. Accordingly, the wave energy converter system in stowed configuration  410  cannot only be shipped or stored in the stowed configuration but can also be towed by means of tug boats during commissioning and decommissioning in the ocean. The stowed configuration  410  provides the system with hydrodynamic properties similar to a familiar pontoon boat, so that the system can be efficiently towed over large distances from a launch port to the deployment location. The ability of the OWEC system to float also eliminates the need for any offshore lifting activities or floating barges or platforms, thus greatly reducing the cost for installation and removal operations. 
       FIG. 4-2  illustrates an intermediate configuration  420  during which legs  131 ,  132 ,  133 , and  134  may be unfolded from the stowed configuration during OWEC installation or folded toward the stowed configuration during removal of the OWEC system. For example, once mooring system  130  and OWEC  110  have reached an installation destination above mooring points  151 ,  152 ,  153 , and  154 , the connection of the mooring latches of legs  131 ,  132 ,  133 , and  134  to platform  120  are released, so that legs  131 ,  132 ,  133 , and  134  can then be unfolded either one at a time or all at once.  FIG. 4-2  shows a configuration  420  in which three legs  131 ,  132 , and  134  are unfolded and one leg  133  remains folded into its stowed position. The unfolding (or folding) may be externally actuated by attaching mooring lines between a tug boat and the end of a leg  131 ,  132 ,  133 , or  134 . However, the actuators may be provided inside braces  121 ,  122 ,  123 , and  124  to support unfolding by pushing the legs  131 ,  132 ,  133 , and  134  from stowed to extended positions. 
     An alternative method of deploying legs  131 ,  132 ,  133 , and  134  may rely on the buoyancy of legs  131 ,  132 ,  133 , and  134 . For example, if each leg  131 ,  132 ,  133 , or  134  is positively buoyant but submerged when platform  120  is positioned above mooring points  151 ,  152 ,  153 , and  154 , releasing the bottom end of the leg from latch points  341 ,  342 ,  343 , or  344  on platform  120  will allow the end of the leg to float outward from platform  120  during deployment. If each leg  131 ,  132 ,  133 , or  134  is negatively buoyant, the bottom end of the leg will sink downward when released from platform  120 . In either case, attachment of the bottom ends of legs  131 ,  132 ,  133 , and  134  to mooring points  151 ,  152 ,  153 , and  154  may be completed in the same manner as when jacking systems in braces  121 ,  122 ,  123 , and  124  are used to deploy legs  131 ,  132 ,  133 , and  134 , but use of buoyancy for leg deployment may avoid the need for jacking systems in one or more of braces  121 ,  122 ,  123 , and  124 . 
     The free ends of legs  131 ,  132 ,  133 , and  134  can be attached to previously installed mooring points  151 ,  152 ,  153 , and  154  located on the ocean floor or on a submerged platform. Mooring points  151 ,  152 ,  153 , and  154  can be installed using a variety of customary offshore techniques like gravity foundations, driven piles, suction caissons, or drilled rock anchors. The technology of choice will typically be determined by local bathymetry of the ocean floor. As mentioned above, the mooring system  130  is very tolerant of inaccuracies in positioning of mooring points  151 ,  152 ,  153 , and  154 , both with regards to lateral and vertical location as well as the orientation of mooring points  151 ,  152 ,  153 , and  154  because legs  131 ,  132 ,  133 , and  134  are independently extensible and because of the rotation freedom of the latches or joints  141 ,  142 ,  143 , and  144  at the bottoms of legs  131 ,  132 ,  133 , and  134 . In order to approximate the leg ends with the mooring points  151 ,  152 ,  153 , and  154 , mooring lines may be used to guide mooring latches on the leg ends into position to mate with mooring points  151 ,  152 ,  153 , and  154 . The outcome of this procedure is the attachment of the wave energy converter to the ocean floor by all four legs, as shown, for example, in  FIG. 1-1 . 
       FIG. 4-3  shows a configuration  430  in which three legs  131 ,  132 , and  134  are attached to mooring points  151 ,  152 , and  154  and the fourth leg is  133  in its stowed position. Three of the four legs with their respective braces are sufficient to stabilize all six degrees of freedom of platform  120 . The configuration  430  may be reached, for example, during commissioning or decommissioning when leg  133  is the last leg to be attached to a mooring point  153  or the first leg to be detached from mooring point  153 . Alternatively, if leg  133  or any associated structure of leg  133  requires maintenance or replacement, leg  133  may be detached from mooring point  153  and raised to its stowed location for maintenance. Legs  131 ,  132 , and  134  at the same time may be extended as described further below to position leg  133  for more convenient maintenance, e.g., to lift stowed leg  133  out of the water. 
     Mooring systems in accordance with examples of the present disclosure may further change configuration during operation of the OWEC, for example, for wave direction alignment, for OWEC depth control, or to place the OWEC in a safe configuration for storms or other ocean-surface events.  FIG. 5-1  shows an example configuration of an OWEC system to alter the compass heading of an OWEC. OWECs, particularly CycWECs, may provide highest-efficiency wave energy conversion when the OWEC is aligned to the current wave direction.  FIG. 5-1  particularly shows OWEC  110  in an operational state that aligns OWEC  110  with a direction  510  of the incoming waves to optimize the power conversion efficiency. This can be achieved by adjusting the lengths of the legs  131 ,  132 ,  133 , and  134 .  FIG. 5-1  particular includes dashed lines representing lengths L 1 A and L 2 A of legs  131  and  132  that will position a generator  112  on the near end of platform  120  at a position  512 A. If the length of leg  131  is extended from length L 1 A to a length L 1 B and the length of leg  132  is reduced from length L 2 A to a length L 2 B, generator  112  shifts laterally to a position  512 B. Changes in the lengths of braces attached to legs  131  and  132  further accommodate or control the shift to position  512 B. On the far end of platform  120 , the length of leg  133  may be increased and the length of leg  134  may be decreased to shift the far end of platform in the opposite lateral direction. Altering the leg lengths in this manner can provide a range of left and right yaw motions that is sufficient to correct alignment of OWEC  110  with the wave direction for almost all of the possible sea states encountered during the operational life of OWEC  110 . In an example configuration, a feedback controller (described further below) may be used to control the alignment and the yaw motion by operating actuators or jacking systems to alter the lengths of legs  131 ,  132 ,  133 , and  134  and braces  121 ,  122 ,  123 , and  124 . In general, legs  131 ,  132 ,  133 , and  134  may be adjusted individually in sequence or simultaneously in parallel to achieve a configuration providing a desired yaw of the OWEC. 
     Adjustment of the lengths of all four legs can be used to optimize the submergence depth, e.g., depth D of  FIG. 1-4 , for energy conversion efficiency, even in the presence of the sea-level changes caused by tidal activity. By optimizing submergence and orientation, the annual wave power extraction can be optimized. 
     Adjustment of the OWEC depth D may also be used for safety.  FIG. 5-2 , for example, illustrates a storm survival configuration in which the mooring system shortens legs  131 ,  132 ,  133 , and  134  to lower OWEC  110  to a deeper depth, where OWEC will not be damaged by wave action. To avoid damage during extreme sea states, platform  120  can be submerged deeper during severe storms. A minimum vertical distance Z S  may be achieved by shortening legs  131 ,  132 ,  133 , and  134  as much as feasible or geometrically possible under the mechanical constraints, which moves platform  120  to the deepest possible position. The most important goal of increasing the submergence is to avoid surfacing of any part of the OWEC system, which would expose it to wind, slamming wave forces and green water impact due to breaking waves, which may cause damage to the structure. Since wave-induced water velocities exponentially decay with submergence depth, this operation can greatly reduce storm-induced loads and thus improve storm survivability of OWEC  110 . Storm survival has been problematic for most of the prior or proposed OWEC systems. 
       FIG. 5-3  illustrates a maintenance configuration for an OWEC system. If legs  131 ,  132 ,  133 , and  134  are extended to maximum lengths, wave energy converter  110 , including platform  120 , may be fully lifted above the ocean surface. In this position, as shown in  FIG. 5-3 , all of the important platform and wave energy converter systems and components may be accessible above water, and the stable platform  120  permits maintenance operations of all important systems and components, for both scheduled and unscheduled maintenance. The ability to move platform to the maintenance configuration may all but eliminate the need to tow OWEC  110  or mooring system  130  back to port for repairs, which is typically far more time consuming and thus costly compared to performing repairs at the deployment site. If any of the leg jacking mechanisms need maintenance or repair, each leg  131 ,  132 ,  133 , or  134  in need of repair may be lifted one at a time, e.g., as shown in  FIG. 4-3 , while platform  120  is in the maintenance configuration as shown in  FIG. 5-3 . With a leg unloaded and securely attached to the side of platform  120 , the leg, the leg jacking mechanism for the leg, the brace  121 ,  122 ,  123 , or  124  attached to the leg, and their mechanical parts can be inspected and serviced as needed. 
       FIG. 6  shows a top view of a line  600  of four-legged OWEC systems that share at least some mooring points  650 . In general, orienting legs  631 ,  632 ,  633 , and  634  of each mooring system to line up with the loads that the OWEC  610  generates may best accommodate the loads and may be the most common orientation. In particular, legs  131 ,  132 ,  133 , and  134  being perpendicular to the main shaft of OWEC  610  provides more direct opposition to reactive forces, with a mooring footprint that is as wide as the length of the platform or OWEC  610 , and mooring points  650  may be ideally oriented along lines perpendicular to the prevailing wave direction at the installation site. In some installations, however, other factors such as secure anchoring of mooring points  650  on the ocean floor may dictate that a positioning of mooring points  650  that is not along the wave fronts of prevailing wave action, so that legs  631 ,  632 ,  633 , and  634  are not be optimally oriented for the reactive loads but are still sufficient for the reactive loads. Also, while many installation locations, may have a prevailing direction for incoming waves, the waves may shift direction at different times of the day or year.  FIG. 6  shows a configuration in which a current wave direction  660  is at a non-perpendicular angle to two rows of mooring points  650 . In the configuration of  FIG. 6 , extending legs  631  and  633  relative to legs  632  and  634  orients OWECs  610  for optimal wave energy conversion, e.g., so that hydrofoils in each CycWEC have lengths perpendicular to the wave direction  660 . 
       FIG. 6  also illustrates how choosing a footprint or separation S between mooring points  650  that is wider than the length of the platform or OWEC  610  has the advantage that in an installation with multiple wave energy converters in a line, the neighboring converters  610  can share mooring points  650  while still being optimally oriented and exposed to the incoming waves without being in the wake of a neighboring converter  610 . Sharing mooring points  650  can greatly reduce the installation cost by decreasing the overall number of mooring points required, and will likely outweigh the minor structural disadvantage experienced by the legs due to their angle with respect to the main shaft. 
       FIG. 7  shows a OWEC system  700  in accordance with another example of the present disclosure. In OWEC system  700 , an OWEC  110  is mounted on a platform  720  of a mooring system  730  having six extensible legs  731 ,  732 ,  733 ,  734 ,  735 , and  736 . Legs  731 ,  732 ,  733 ,  734 ,  735 , and  736  are extensible to independently change lengths as needed to define a configuration of mooring system  730 , and each leg  731 ,  732 ,  733 ,  734 ,  735 , or  736  may contain a jacking or actuation system such as system  200  described above with reference to  FIG. 2 . Legs  731  and  736  have top ends connected near a first corner of platform  720  using joints, e.g., Cardan joints, that allow independent rotations of legs  731  and  736  about two axes. Leg  732  has a top end connected near a second corner of platform  720  using a joint, e.g., a Cardan joint, that allows independent rotation of leg  732  about two axes. Legs  733  and  734  have top ends connected near a third corner of platform  720  using joints, e.g., Cardan joints, that allow independent rotations of legs  733  and  734  about two axes. Leg  735  has a top end connected near a fourth corner of platform  720  using a joint, e.g., a Cardan joint, that allows independent rotation of leg  735  about two axes. Other joints, e.g., further Cardan joints, or mooring latches connect the bottom end of leg  731  to a mooring point  751 , the bottom ends of legs  732  and  733  both to a mooring point  753 , the bottom end of leg  734  to a mooring point  754 , and the bottom ends of legs  735  and  736  both to a mooring point  756 . The four mooring points  751 ,  753 ,  754 , and  756  may be attached to the ocean floor and nominally arranged at the vertices of a parallelogram, but the ability of legs  731 ,  732 ,  733 ,  734 ,  735 , and  736  to change lengths and the rotational freedom of joints attaching legs  731 ,  732 ,  733 ,  734 ,  735 , and  736  to mooring points  751 ,  753 ,  754 , and  756  allows mooring system  730  to adapt significant variation in the positioning of mooring points  751 ,  753 ,  754 , and  756 . 
     Even though each of the joints connecting legs  761 ,  762 ,  763 ,  764 ,  765 , and  766  to platform  720  and mooring points  751 ,  753 ,  754 , and  756  allow two degrees of freedom of rotations, fixing the lengths of legs  761 ,  762 ,  763 ,  764 ,  765 , and  766  provides a stable/rigid configuration of mooring system  730 . In comparison to OWEC system  100  described above, mooring system  730  employs six extensible structural members  761 ,  762 ,  763 ,  764 ,  765 , and  766 , while mooring system  130  employed eight extensible structural members  131 ,  132 ,  133 ,  134 ,  121 ,  122 ,  123 , and  124 . The pairing of legs, e.g., legs  732  and  733  or  735  and  736 , attached to the same mooring point, e.g., mooring point  753  or  756 , is structurally similar to a leg and a brace. In the illustrated configuration, each pair of legs  732  and  733  or  735  and  736  converges laterally toward mooring point  753  or  756 , providing lateral bracing. The converging of each leg  761 ,  762 ,  763 ,  764 ,  765 , and  766  from the wider separations of mooring points  751 ,  753 ,  754 , and  756  toward more narrowly space connections on platform  720  provides torque bracing. Although the geometry of mooring system  730  differs from the geometry of mooring system  130 , proportional changes in the lengths of legs  761 ,  762 ,  763 ,  764 ,  765 , and  766  may change the elevation of OWEC  110  from an above water position to a range of operational depths for wave energy conversion to a deep submersion for storm safety in a manner similar to described above with reference to  FIGS. 5-2 and 5-3 . Additionally, increasing or decreasing the lengths  731  and  736  and/or  733  and  734  (while correspondingly decreasing or increasing the lengths of legs  732  and/or  735 ) may yaw OWEC  110  right or left, e.g., to position OWEC  110  according to a wave direction. 
       FIG. 8  shows a OWEC system  800  in accordance with an example of the present disclosure in which an OWEC  110  is mounted on a platform  820  of a mooring system  830  having five extensible legs  831 ,  832 ,  833 ,  834 , and  835 . Legs  831 ,  832 ,  833 ,  834 , and  835  are extensible to independently change lengths as needed to define a configuration of mooring system  830 , and each leg  831 ,  832 ,  833 ,  834 , or  835  may contain a jacking or actuation system such as system  200  described above with reference to  FIG. 2 . Legs  831  and  832  have top ends connected near a first and second corners at one end of platform  820 . Legs  833  and  834  have top ends connected near a third corner at an opposite end of platform  820 . Leg  735  has a top end connected near a fourth corner at the opposite end of platform  720 . The top ends of legs  831 ,  832 ,  833 ,  834 , and  835  are connected to platform  820  using joints, e.g., Cardan joints, each of which supports compress and tension but allows independent rotation of the attached leg about one or two axes. Other similar joints, e.g., further Cardan joints or latches, connect the bottom end of legs  831  and  835  to a mooring point  851 , the bottom ends of legs  832  and  833  both to a mooring point  852 , and the bottom end of leg  834  to a mooring point  854 . The three mooring points  851 ,  852 , and  854  may be nominally arranged at the vertices of a horizontal triangle, but the ability of legs  831 ,  832 ,  833 ,  834 , and  835  to change lengths allows mooring system  830  to adapt to significant variations in the positioning of mounting points  851 ,  852 , and  854 . 
     OWEC system  800  differs from OWEC system  100  and  700  in that OWEC system  800  only needs three mooring points  851 ,  852 , and  854  for anchoring of OWEC  110 . In contrast, OWEC systems  100  and  700  each use four mooring points. Additionally, in one variation of OWEC system  800 , instead of extensible structural members  833  and  835  connecting to mooring points  852  and  851 , the bottoms of structural members  833  and  835  may instead be respectively connect to legs  832  and  831  as extensible braces, making mooring system  830  a three-legged mooring system. Mooring system  830  has only five extensible structural elements  831 ,  832 ,  833 ,  834 , and  835 , and fixing and holding the lengths of structural elements  831 ,  832 ,  833 ,  834 , and  835  constrains only five degrees of freedom of motion of OWEC  110  if all of the joints connecting structural members  831 ,  832 ,  833 ,  834 , and  835  to platform  820  and mooring points  851 ,  852 , and  854  provide two degrees of freedom to swing the element about the joint. To restrict all six degrees of freedom of platform  820 , some of the joints are not Cardan joints with two degrees of swing rotation allowed but have only one degree of rotation, e.g., one or more of the joints may be a hinge or pivot structure. The configuration of mooring system  830  or OWEC system  800  may, however, be changed, for example, by expanding or contracting the lengths of structural elements  831 ,  832 ,  833 ,  834 , and  835  to raise or lower OWEC  110 . The orientation or compass heading of OWEC  110  may be changed by extending or shortening the lengths or structural elements connected to diagonally opposing corners of platform  820 . 
       FIG. 9  is a block diagram of a OWEC system  900  including a control system  910  for an OWEC  920  on a mooring system  930  that is installed in the ocean to convert wave energy to electrical energy. Control system  910  may include a computer that is either at the installation location or remote from wave energy converter installation. Control system  910  generally receives sensor signals from sensors  940  and generates control signals that control the operation of OWEC  920  and mooring system  930 . Sensors  940  may generally measure weather and ocean conditions and the performance and operating parameters of OWEC  920  or mooring system  930 . For example, sensors  920  may measure current amplitudes and propagation direction of incoming waves at the installation site, conversion efficiency indicated by amplitudes of waves after passing by OWEC  920 , the orientation, depth, and rotational frequency of OWEC  920 , and the power output of OWEC  920 . 
     In the illustrated configuration, control system  910  includes a management module  912 , an operation module  914 , a safety module  916 , and a maintenance module  918 . Management module  912  may be configured to provide a user interface that allows a human manager to monitor and manage the operation of OWEC  920  and the energy that OWEC  920  produces. Operation module  914  may be configured to automatically control operation of OWEC  920  and mooring system  930 . In particular, operation module  914  may monitor the operating parameters and performance of OWEC  920 , which may be measured by sensors  940 , and based on the sensor input control or actuate a pitch or pitching cycle of hydrofoils in OWEC  920 , control jacking or actuation systems of mooring system  930  to provide OWEC  920  with a depth and orientation that optimizes power conversion efficiency for the current ocean conditions. Safety module  916  may be configured operate mooring system  930  to lower OWEC  920  to a sufficiently deep depth to withstand storms or exceptionally high seas. Maintenance module  918  may be configure to raise OWEC  920  or mooring system  930  into a configuration suitable for maintenance or repair procedures. 
     Each of modules disclosed herein may include, for example, hardware devices including electronic circuitry for implementing the functionality described herein. In addition or as an alternative, each module may be partly or fully implemented by a processor executing instructions encoded on a machine-readable storage medium, e.g., a non-transient media, such as an optical or magnetic disk, a memory card, or other solid state storage containing instructions that a computing device can execute to perform specific processes that are described herein. 
     Although particular implementations have been disclosed, these implementations are only examples and should not be taken as limitations. Various adaptations and combinations of features of the implementations disclosed are within the scope of the following claims.