Patent Issued for Cantilevered tension-leg stabilization of buoyant wave energy converter or floating wind turbine base (USPTO 11131287)
- Oct 19, 2021 11:27 pm GMT
2021 OCT 19 (NewsRx) -- By a
The assignee for this patent, patent number 11131287, is
Reporters obtained the following quote from the background information supplied by the inventors:
“Ocean waves are produced by offshore winds. Waves near the ocean surface typically have 5-10 times the energy density (kw/m2) of offshore winds that produce such waves. The offshore wind-energy resource has higher energy content and is more consistent than terrestrial winds. Offshore wind farms are now the major source of renewable power capacity additions in
“Wave energy is also a huge global renewable energy resource but, despite its higher energy density, lags offshore wind development with no large utility-scale-wave energy farms yet commercially deployed. This is due both to the profusion of distinctly different proposed means of converting ocean waves into electrical power and to the high capital cost (CapEx) per installed megawatt (MW) of those primitive early generation WECs which have been scaled-up and ocean-deployed to date. WECs also have unique marine design challenges. Ocean wave energy is most concentrated on the ocean surface and decreases exponentially with depth, thereby making it most desirable to deploy WECs on the surface. Wave energy, however, is proportional to wave height squared. A WEC designed for peak output in 4-meter (significant wave height) seas must survive (or avoid) 16 times higher structural loads during occasional severe winter storms that produce wave heights 4 times higher than the 4-meter design wave height.
“By combining WECs and offshore wind turbines, whether of the fixed or floating base type, the cost of the combined power output can potentially be lowered by not only using a common base but by also using common mooring systems, inter-array and array-to-shore sea-cables, and using common operating and maintenance resources. Numerous combined FWT-WEC devices have been proposed but most utilize the combination to improve or mask the poor economic prospects of WECs with intrinsically high CapEx/MW. Combining a specific economically viable WEC design with an FWT can be especially promising if both can synergistically share an affordable, effective wave-and-wind-motion-stabilized base or frame with improved motion stabilization as achieved with the present disclosure.
“All WECs require at least one first active body, typically a buoyant float or flap, and a second reaction body or mass, typically a second floating body, a frame, a base, a platform, or a shoreline or seabed-affixed frame, tower, or base, or the seabed itself. Wave energy is captured from the wave-induced relative motion, between the first active body and the second reaction body that drives a power take-off (PTO) device such as an electric generator, a hydraulic pump, or an air turbine.
“It is highly desirable to stabilize FWTs against wave, wind or wind-gust-induced pitching motions. Most current and proposed FWTs use horizontal axis turbines. Significant deviation of their rotational axes from the oncoming, substantially-horizontal wind direction produces a reduction in wind turbine energy capture efficiency. Vertical heaving or lateral surging movement of the FWT base will generally result in a lesser, though still significant, reduction in wind turbine efficiency.
“WEC wave energy capture efficiency is also substantially reduced by unwanted wave or wind-induced heave (vertical), surge (lateral), and/or pitching (rotational) motion of the WEC reaction body (which can be a floating or semi-submerged frame, base, raft, or platform). Such unwanted wave or wind-induced reaction-body motion will substantially reduce the relative motion between a WEC’s first or active body (such as a flap or a float) and the WEC’s reaction body. Because captured wave energy is the product of the relative motion between the active and reactive body times the resistive force (applied by the generator or other power take-off means) between the two (or more) bodies, any reduction in the relative motion between the bodies (caused by wave-induced heave, surge, and/or pitching motion of the reactive body) reduces wave energy capture.
“One way of motion-stabilizing the floating or semi-submerged base or reaction body of FWTs or WECs is to make them massive. This can be done by the sheer weight of the metal (or concrete) used to fabricate them which can be further enhanced by integral or attached water ballast tanks or drag plates that capture or entrain additional seawater mass (as shown in FIG. 1 Semi-Submersible). Unfortunately, the fabricated marine steel, aluminum, fiberglass, and/or concrete utilized in such massive FWT or WEC bases often makes the bases more massive and expensive than their wind turbines or WEC floats or flaps that do the actual energy capture work.
“Wind and wave energy resources may be free, but the capital required to capture and convert these renewable resources into usable power is not free. The cost of ocean-energy-produced power (often referenced as the LCOE or Levelized Cost of Energy) is primarily determined by the installed capital cost or expense (CapEx)/unit of output or CapEx/MW) required to capture, convert and deliver it. Even FWT and WEC operating and maintenance expenses are a relatively fixed percent of their CapEx.
“The seabed itself can also be utilized as part of the base or reaction-body mass to stabilize FWT and WEC bases or reaction bodies. The Tension Leg Platform shown in FIG. 1 utilizes three tensioned cables affixed to the seabed to stabilize a buoyant, fully-submerged platform to which a wind turbine tower is mounted. The seabed is certainly massive and immobile, but the large buoyant platform with its three tensioned legs are not without substantial mass or CapEx. The three tensioned leg cables and their secure attachments to the seabed are also not without costs. These tension legs must be able to withstand tidal changes in platform-submerged depth and severe “snap loads” from occasional, but severe, sea conditions that produce waves up to 15 meters in height that slacken and then suddenly re-tension these cables.
“Objects of the Disclosure
“The descriptions and operating principles of the present disclosure focus primarily on the stabilization of FWT bases, WEC floating bases, or combined FWT and WEC devices that utilize a common floating, semi-submerged, or buoyant base by use of at least one up-sea, substantially submerged, cantilevered mooring beam connected to a substantially submerged, buoyant mooring buoy connected to the seabed by at least one tensioned leg or cable. The disclosure also includes application of these components and principles to stabilize other buoyant, semi-submerged, moored platforms or bases from undesirable wave or wind-induced motions.
“One object of the disclosure is to provide an effective means to motion-stabilize or motion control an FWT base, WEC base, a combination FWT-WEC base or other floating platform or base from undesirable wave or wind-induced motion while minimizing the structural mass and CapEx of such platform or base. Another object of the disclosure is to provide a mooring system for a motion-stabilized base that enhances such base stabilization against wave or wind-induced motion. Yet another object of the disclosure is to effectively utilize the gyroscopic effect of a rotating FWT to further stabilize the FWT or combined FWT-WEC base against wind gust or wave-induced aft-ward pitching or other undesirable motion.
“Another object of the disclosure is to provide a base mooring system that allows a WEC, FWT, combined FWT-WEC base, or other floating base to pivot or swivel in a horizontal plane around a mooring point or buoy to self-orient or weather-vane into oncoming wave fronts to either improve base stabilization or to increase WEC capture efficiency by allowing the WEC to intercept maximum oncoming wave-front width. Such a mooring system concurrently enhances the motion stabilization of such a base.
“Yet another object of the disclosure is to provide a base mooring system that allows a WEC, FWT, or combined FWT-WEC base to pivot or swivel in a horizontal plane around a mooring point or buoy to self-orient or weather-vane into oncoming wave fronts or wind gusts while extending the fore-to-aft dimension of such a single or multiple-base float by utilizing the disclosed mooring beam. Such an elongated fore-to-aft dimension spans a significant portion of oncoming wave lengths and thereby reduces fore-to-aft pitching of such a base or frame.
“A further object of the disclosure is to provide a WEC, FWT, combined WEC-FWT base or other floating-base mooring system that provides self-orientation of such base into oncoming wave or wind about a mooring or pivot point or buoy, and concurrently provides base motion stabilization by utilizing one or more tensioned cables between the seabed and such mooring or pivot point. Such a base is pivotably connected to such a mooring point by a semi-rigid mooring beam that is rigidly connected to such a base.”
There is additional summary information. Please visit full patent to read further.
In addition to obtaining background information on this patent, NewsRx editors also obtained the inventors’ summary information for this patent: “In one aspect of the disclosure, it is desirable to have a floating base or platform such as a WEC base, FWT base, or combination WEC-FWT base motion-stabilized or controlled against wave or wind-induced motions. It is further desirable to have such bases be as light-weight and inexpensive (low CapEx) as possible. Multiple tensioned leg or cable connections between the base and the seabed can provide supplemental seabed stabilization mass for such a base but will restrict desirable self-orientation (weather-vanning) of the base and any attached WEC device. WEC self-orientation is desirable because it allows wide-beam WEC surface floats to intercept maximum wave-front-containing wave energy per unit of float volume, mass, and cost (CapEx). Multiple tension legs attached directly to such floating or semi-submerged bases are also of limited effectiveness against lateral motion stabilization induced by wave surge (lateral) forces or lateral winds and gusts. To maximize wave-energy-capture efficiency, WECs must absorb a majority of both heave and surge wave-energy components, each of which is equal to exactly half of the total wave energy in deep water, which makes WEC base lateral motion stabilization essential. Multiple tension-leg-moored bases or floats do not compensate for tidal changes in the still water level or line (SWL), which change can change the submerged depth of such bases and significantly reduce the efficiency and effectiveness of WECs that utilize such bases as their reaction body.
“The disclosure provides a relatively low-mass, low-CapEx, effectively motion-stabilized or motion controlled floating or semi-submerged base or platform that can be utilized as an FWT base, WEC base, or combination FWT-WEC base. Undesirable, excessive base-stabilization mass is reduced by pivotably connecting the base to an up-sea submerged mooring buoy or pivot point that utilizes an elongated mooring beam rigidly attached (cantilevered) to the base. The mooring buoy is attached to the seabed by at least one tensioned leg (or cable). The up-sea horizontal pivoting attachment between the elongated mooring beam and the submerged mooring ball or pivot point allows any WEC surface floats on the motion-stabilized, low-mass base to remain self-oriented (weather vanning) into oncoming wave fronts, which maximizes intercepted wave-front width, and may allow self-orientation of one or more FWTs on such a base. The elongated, cantilevered mooring beam of the disclosure also allows for significant tidal SWL adjustment because the mooring beam length can be substantially longer than the tidal range, which is particularly necessary for WECs.
“Because the mooring buoy or pivot point is submerged substantially below the SWL and restricted from upward vertical motion by at least one seabed affixed tensioned cable, wave-(and wind or wind gust)induced lateral forces applied against the base, and any FWT or WEC attached to such base, produce an aft-ward base-pitching moment about a the mooring buoy pivot point. Waves under the base also concurrently attempt to heave the base upward that produces an opposing forward-pitching moment. By judiciously selecting the mooring beam length and the mooring buoy submerged depth, wave-surge-(or wind-)induced aft pitching can be substantially or fully cancelled by wave-heave-induced forward pitching.
“The disclosure has additional novel and unique synergistic advantages when a combined WEC-FWT is combined with the disclosed stabilized base. The rotational inertia of an FWT on a base enhances base stabilization against pitch (gyroscopic effect). Adjustment of the mooring beam length or base seawater ballast allows optimum FWT horizontal axis adjustment. The mass of the FWT on top of its tower substantially increases the moment of inertia and increases the natural frequency of the combined FWT and base substantially reduces wave-induced aft pitching, which, in turn, increases the WEC float-to-base relative motion and energy-capture efficiency, especially for large waves and long-wave periods where most WECs struggle.
“Base pitch, heave stability and attitude of the disclosed apparatus can be further enhanced by admission or discharge of seawater ballast from at least one cavity within the base or by using one or more substantially horizontal or vertical-plane drag plates affixed to, or extending from, the base. An optional inclined shoaling plane mounted on the mooring beam, preferably such that it does not interfere with WEC float rotation, also enhances base motion stability while increasing wave height and WEC capture efficiency. Embodiments of the present disclosure utilize a concave WEC float aft wall, which together with any lower extension thereof, and with optional WEC float side plate aft extensions, further reduce base aft pitching. This concave rear float wall, approximately concentric about the drive arm pivot point, fully eliminates or substantially reduces generation of any aft or back waves during the float’s movements, which back wave generation would otherwise substantially reduce WEC wave-energy capture efficiency.”
The claims supplied by the inventors are:
“1. A motion-stabilizing and mooring device comprising: a first buoyant body, wherein the first buoyant body is floating, semi-submerged, or submerged body, and has a center of buoyancy, wherein the first buoyant body, including appendages affixed to the first buoyant body, is stabilized against wave, wind, or wind-gust-induced motion, wherein the first buoyant body is selected from a group consisting of a base, a platform, a raft, a barge, a buoy, other buoyant bodies and combinations thereof; a second buoyant body, wherein the second buoyant body is substantially submerged, has a center of buoyancy located substantially below the center of buoyancy of the first buoyant body, and is located substantially up-sea, upwind or forward of the first buoyant body relative to oncoming waves or wind, wherein the second buoyant body is selected from the group consisting of a buoy, a spar, a mooring body and combinations thereof; and, at least one mooring beam affixed to, and extending from, the first buoyant body and connected to the second buoyant body at a connection point in such manner as to allow the mooring beam to pivot in a substantially horizontal or substantially vertical plane about the second buoyant body; and, at least one tensioned connection member selected from a group consisting of a cable, a line, a leg, a pole, a piling, a beam, a truss, a protrusion extending upwardly from the seabed and combinations thereof, wherein the at least one tensioned connection member is secured to the second buoyant body comprising at least one seabed-affixed tensioned member, which tensioned connection member results in a tensioned load in the at least one tensioned connection member produced by the buoyancy of the second buoyant body, the load being increased or decreased by the wave or wind-induced forces applied to the second buoyant body from the first buoyant body through the mooring beam.
“2. The device of claim 1 wherein the connection between the at least one mooring beam and the second buoyant body and/or the connection between the second buoyant body and the at least one tensioned connection member is a mooring body horizontal plane pivotable connection is structured to allow the first buoyant body to self-orient or weathervane substantially normal to an oncoming or a prevailing wave or a wind direction, and/or wherein the connection is a substantially vertical pivotable or vertically translatable connection structured to allow the first buoyant body to remain at a relatively constant orientation relative to the still water line (SWL) or at a relatively constant submerged depth relative to the SWL as the SWL rises or falls with tidal changes.
“3. The device of claim 1 wherein the first buoyant body is a base or a frame of a wave energy converter (WEC) for converting the energy of waves into electrical power; or pressurized fluid, or wherein the first buoyant body is a base or a frame of a floating wind turbine (FWT) for converting the energy of offshore winds into electrical power or useful work, or wherein the first buoyant body is a common base or frame of a combined WEC-FWT, wherein the combined WEC-FWT has a WEC component and a FWT component.
“4. The device of claim 3 wherein the first buoyant body is the base or frame of a WEC having at least one base pivot connection point or pivot axis located substantially below the SWL, wherein the WEC further comprises: at least one buoyant float body having a center of buoyancy located substantially forward or up-sea of the base pivot point and a front face oriented substantially towards oncoming wave fronts, the at least one buoyant float body being movably connected to the base or frame at a base pivot connection point or pivot axis by at least one swing or drive arm that controls an orientation and path of a wave-induced relative motion between the at least one buoyant float and the base or frame; and, at least one power take-off or PTO apparatus secured to, or within, the base or frame, wherein the at least one power take-off or PTO apparatus is driven by at least one force generated by the wave-induced relative motion between the at least one buoyant float and the base or frame through the at least one swing or drive arm, and wherein the at least one power take-off or PTO apparatus may also drive motion of the at least one buoyant float during certain portions of each wave cycle.
“5. The device of claim 4 wherein the at least one buoyant float body, and any adjacent buoyant float bodies, have front face oriented to, or self-orienting substantially parallel to, prevailing or oncoming wave fronts, wherein the at least one buoyant float body, and any adjacent buoyant float bodies, have a wave-front width or beam substantially exceeding a fore-to-aft dimension of the at least one buoyant float body, and any adjacent buoyant float bodies, wherein the fore-to-aft dimension excludes appendages or extensions, and wherein the at least one buoyant float body has an arcuate length and has a rear or aft side substantially concave and concentric about an at least one base pivot connection point or pivot axis for at least a major portion of the at least one buoyant body’s arcuate length.
“6. The device of claim 3 wherein a submerged portion of the first buoyant body contains at least one elongated, substantially vertical spar that may have seawater, gravity weights, or fixed or adjustable ballasts secured or affixed to the vertical spar, wherein the first buoyant body is constructed such that its center of buoyancy is located substantially above its center of gravity.
“7. The device of claim 1 wherein the horizontal and vertical distance between the center of buoyancy of the first buoyant body and the center of buoyancy of the second buoyant body located forward and below the first buoyant body, are selected such that a moment produced by the vertical, upward, wave-heave-induced forces acting on the first buoyant body, and any attachments secured to the first buoyant body, are at least partially countered by an opposing moment produced by lateral wave-surge-induced forces plus lateral wind-induced forces, if lateral wind-induced forces are present.
“8. The device of claim 1 wherein the at least one mooring beam, its connection to the first buoyant body, or its connection to the at least one tensioned connection member that connects the second buoyant body to the seabed is constructed to have a flex, spring, or energy absorption or storage sufficient to reduce shock or snap loadings on the at least one tensioned connection member caused by wave or wind-induced forces upon the first buoyant body and transmitted through the at least one mooring beam, the second buoyant body, to the at least one tensioned connection member and to the seabed.
“9. The device of claim 1 further comprising a substantially vertical mooring body shaft, wherein the submerged depth of the second buoyant body can be substantially vertically adjusted to compensate for tidal changes in the SWL or sea conditions, wherein the second buoyant body submerged depth can be adjusted vertically with a motor connected to the second buoyant body, wherein the second buoyant body submerged depth can be adjusted vertically via a low-speed, self-movement of the second buoyant body vertically on the substantially vertical mooring body shaft without the motor assistance, or by adjusting the length of the at least one tensioned connection member connected to the second buoyant body without the motor assistance.
“10. The device of claim 3, wherein the first buoyant body is the base or the frame of a combined WEC-FWT and comprises a wind turbine rotor having an axis of rotation and a generator, wherein a gyroscopic stabilization effect of the wind turbine rotor and the generator about the axis of rotation supplements the motion stabilization of the first buoyant body against wave or wind-induced motion provided by the at least one seabed-attached, tensioned connection member connected to the first buoyant body through the second buoyant body and the at least one mooring beam or structural member.
“11. The device of claim 3, wherein the first buoyant body is the base or the frame of a combined WEC-FWT, wherein wave-induced forces acting upon the combined WEC-FWT base or frame oppose wind-induced forces acting upon the base or frame to improve motion stabilization of, and/or self-orientation of, the combined WEC-FWT base or frame.
“12. The device of claim 3, wherein the first buoyant body is the base or the frame of a combined WEC-FWT, wherein the FWT component of the combined WEC-FWT has an FWT tower having a FWT tower head mass and a FWT tower mass, whereby the FWT tower head mass and the FWT tower mass, with or without additional ballast or gravity mass added to lower portions of the combined WEC-FWT, substantially increase the moment of inertia of the FWT component combined with the first buoyant body of the WEC component of the combined WEC-FWT about their combined center of gravity, which increased moment of inertia increases a natural frequency period of the combined FWT with the first buoyant body with respect to wave-induced aft pitching, which thereby increases the relative motion and energy capture efficiency of the WEC component, especially during large or long-period waves.
“13. The device of claim 3 wherein energy absorbing, spring-like, or other energy storage elements within the at least one mooring beam or structural member, within the at least one mooring beam or structural member’s connection to the first buoyant body, or along the at least one seabed-attached tensioned connection member are structured to delay pitch, heave, or lateral rebound of the first buoyant body from a prior wave until a subsequent wave is at least partially applying pitch, heave, or lateral forces against the first buoyant body.”
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