Patent Publication Number: US-2017363066-A1

Title: Methods and Systems for Electrical Isolation in an Offshore Power Generation Plant

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of U.S. Patent Application No. 62/351,550 filed Jun. 17, 2016 entitled METHODS AND SYSTEMS FOR ELECTRICAL ISOLATION IN AN OFFSHORE POWER GENERATION PLANT, the entirety of which is incorporated by reference herein. 
     This application is related to U.S. Provisional Patent Application No. 62/351,528, entitled “Systems and Methods for Offshore Power Generation Using Airborne Power Generating Craft”, U.S. Provisional Patent Application No. 62/351,541, entitled “Systems and Methods for Offshore Power Generation Using Airborne Power Generating Craft Tethered to a Floating Structure”, U.S. Provisional Patent Application No. 62/351,547, entitled “Methods and Systems of Maintaining an Offshore Power Plant”, and U.S. Provisional Patent Application No. 62/351,552, entitled “Method and Systems for Maintaining an Offshore Power Plant Having Airborne Power Generating Craft”, all of which are filed on an even date and have a common assignee herewith, the disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     Field of Disclosure 
     The disclosure relates generally to offshore power generation, and more particularly, to tethered wind turbine systems. 
     Description of Related Art 
     This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as an admission of prior art. 
     A wind turbine converts the energy of moving air into electricity or other forms of energy. A common type of wind turbine system includes an electrical generator driven by rotor blades mounted in a rotatable manner near an upper end of a vertical support tower. The rotor may be rotated relative to the tower as the wind direction changes such that the blades of the rotor are maintained perpendicular to the wind. These windmill-type wind turbine systems have become popular on land in regions that have open space and sufficient average wind velocities, and have also been adapted for use in offshore locations. Offshore locations offer the benefit of open space and potentially higher average sustained wind speeds. 
     Concepts for deeper water installations that are currently under development are mostly derived from configurations for offshore oil well rigs to include floating platforms. Accordingly, such concepts typically require large cranes for erection of the towers and turbines and are not optimal for wind turbines because of the large aerodynamic force in the direction of the wind as well as forces associated with dynamics from the angular momentum of the turbine blades. Furthermore, wind and wave forces cause coupled motions of the support tower and the rotor blades, resulting in greater structural dynamic loads, deflections and stresses upon the wind turbine system. The options of the prior art include large costly structures, with masses and/or dimensions often many times that of the wind turbine they are designed to support. For example, a typical offshore wind turbine system may have a height of approximately 100 meters from the sea surface with a weight of hundreds of tons. 
     One solution to the high cost of installation of wind turbines is an apparatus that is tethered to a fixed point. The apparatus generates electrical power by harnessing the wind in some manner. An example of a tethered wind turbine system is illustrated in  FIG. 1  and is indicated generally by reference number  10 . System  10  includes a wing or blade  12  fastened to a base  14  using a tether line  16 . The blade  12  is shaped to move generally perpendicular to the direction of the blowing wind W in a path, such as circular path  18 . The blades may be shaped to perform lift when wind W is passed over it. As the blade moves, propellers  20  mounted on the blade rotate and cause electrical power to be generated by motor/generators  22 , to which the propellers are rotatably mounted. The power so generated is transmitted through tether line  16 . Blade  12  may be raised and lowered by extending or retracting tether line  16 , and may be brought to rest on a mount or cradle  24 , which may be an integral part of base  14 . System  10  may be launched from its cradle using the motor/generators  22  in a motoring mode. Power transmitted to the motor/generators  22  is used to drive the propellers  20  in the motoring mode. Once at the desired altitude, and when wind velocities are sufficiently high and/or constant, system  10  may autonomously shift to a self-sustained state of flight using lift generated by blade  12 , and the motor/generators  22  generate power as previously described. The motor/generators  22  preferably are operated in a motoring mode to control the descent of blade  12  as the blade is returned to rest on cradle  24 . System  10  as described has been developed by Makani Power, Inc. of Alameda, Calif. 
     Because system  10  requires no heavy vertical support tower, the mass of system  10  is significantly less than a similarly rated conventional wind turbine system—perhaps as much as 90% less. Additionally, system  10  may be employed at altitudes of 300 meters or more, potentially harnessing the stronger and more consistent winds there. Such altitudes simply are not commercially accessible by conventional systems using a vertical support tower. At these high altitudes, 85% of the United States can offer viable wind resources compared to the 15% of the United States accessible with conventional wind turbine technology. More importantly, because of the significant weight reductions and the potential for high altitude deployment, system  10  may be advantageously deployed in offshore waters, opening up a resource which is four times greater than the entire electrical generation capacity of the United States. 
     Current solutions for implementing system  10  offshore require placing base  14  on a semi-submersible structure that is secured to the seafloor with multiple anchoring cables. Such a solution still requires transporting and anchoring the semi-submersible structure, and these activities may reduce the commercial feasibility of system  10 . There is a need to reduce the cost of installation and to reduce the capital expenditures required to install wind power at sea, or over a body of water. There is also a need for solutions which enable installations in deeper water which are cost effective and suitable for the harsh deep water conditions. Therefore, it would be desirable to provide an offshore wind turbine system that can easily be installed in deep water locations and that minimizes or eliminates requirements for a foundational support structure at the water&#39;s surface. 
     SUMMARY 
     The present disclosure provides a method of generating power. An airborne power generating craft is connected to an anchor using a tether line. The anchor is secured to an underwater floor. Power is generated based on movement of the airborne power generating craft in response to a wind force. The tether line is maintained at a constant length between the airborne power generating craft and the anchor as the airborne power generating craft moves in response to the wind force. The airborne power generating craft is connected to an electrical transmission system through at least part of the tether line. The generated power is transmitted to the electrical transmission system. A condition is sensed in which transmitting power to the electrical transmission system is not desired. The airborne power generating craft is electrically isolated to prevent power from being transmitted from the airborne power generating craft to the electrical transmission system. 
     The present disclosure also provides an offshore power generation system including an airborne power generating craft. A tether line is connected at a first end to the airborne power generating craft. The second end of the tether line is attached to an anchor secured to an underwater floor. The length of the tether line between the airborne power generating craft and the anchor is constant. An electrical transmission system is connected to the airborne power generating craft through the tether line. The electrical transmission system transmits power generated by the airborne power generating craft. A sensor senses a condition in which transmitting power to the electrical transmission system is not desired. An electrical isolation mechanism prevents power from being transmitted from the airborne power generating craft to the electrical transmission system in response to the sensed condition. 
     The foregoing has broadly outlined the features of the present disclosure so that the detailed description that follows may be better understood. Additional features will also be described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the disclosure will become apparent from the following description, appending claims and the accompanying drawings, which are briefly described below. 
         FIG. 1  is a side elevational view of a known tethered wind turbine system. 
         FIG. 2  is a side elevational view of a tethered wind turbine system according to disclosed aspects. 
         FIG. 3  is a perspective view of a portion of the tethered wind turbine system of  FIG. 2  according to disclosed aspects. 
         FIG. 4  is a detail view of a portion of the tethered wind turbine system of  FIGS. 2 and 3  according to disclosed aspects. 
         FIG. 5  is a cross-section view of the tether taken along line  5 -- 5  in  FIG. 2  according to disclosed aspects. 
         FIG. 6  is a detail view of a portion of an anchor pile shown in  FIG. 2  according to disclosed aspects. 
         FIG. 7  is a detail view of a portion of a tether shown in  FIG. 2  according to disclosed aspects. 
         FIG. 8  is a plan view of a wind farm according to disclosed aspects. 
         FIG. 9  is a side elevational view of a tethered wind turbine system according to disclosed aspects. 
         FIG. 10  is a perspective view of an offshore support vessel according to disclosed aspects. 
         FIG. 11  is a side elevational view of a tethered wind turbine system according to disclosed aspects. 
         FIG. 12  is a side elevational view of a tethered wind turbine system according to disclosed aspects. 
         FIG. 13  is a schematic diagram of a control system according to disclosed aspects. 
         FIG. 14  is a side elevational view of a buoy according to disclosed aspects. 
         FIG. 15  is a side elevational view of a method of transporting a tethered wind turbine system according to disclosed aspects. 
         FIG. 16  is a method according to aspects of the disclosure. 
         FIG. 17  is a method according to aspects of the disclosure. 
         FIG. 18  is a method according to aspects of the disclosure. 
         FIG. 19  is a method according to aspects of the disclosure. 
         FIG. 20  is a method according to aspects of the disclosure. 
         FIG. 21  is a method according to aspects of the disclosure. 
         FIG. 22  is a method according to aspects of the disclosure. 
     
    
    
     It should be noted that the figures are merely examples and no limitations on the scope of the present disclosure are intended thereby. Further, the figures are generally not drawn to scale, but are drafted for purposes of convenience and clarity in illustrating various aspects of the disclosure. 
     DETAILED DESCRIPTION 
     To promote an understanding of the principles of the disclosure, reference will now be made to the features illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications, and any further applications of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. For the sake of clarity, some features not relevant to the present disclosure may not be shown in the drawings. 
     At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims. 
     As one of ordinary skill would appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name only. The figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. When referring to the figures described herein, the same reference numerals may be referenced in multiple figures for the sake of simplicity. In the following description and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus, should be interpreted to mean “including, but not limited to.” 
     The articles “the,” “a” and “an” are not necessarily limited to mean only one, but rather are inclusive and open ended so as to include, optionally, multiple such elements. 
     As used herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numeral ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and are considered to be within the scope of the disclosure. 
     As used herein, terms such as “offshore”, “seafloor”, “subsea”, “underwater”, and “water” are to be interpreted to refer to or describe any body of water, including oceans, lakes, reservoirs, seas, and rivers. 
     As used herein, the terms “electricity” and “power”, when referring to the generation, transmission, and storage thereof, may be used interchangeably as is known in the art. 
     The disclosed aspects include a power generation plant having one or more tethered wind turbine systems, coupled with appropriate electrical infrastructure and energy storage technology, which may be configured to power new or existing developments. Such developments are described herein and may include offshore and/or onshore developments. 
       FIG. 2  illustrates a power generation plant  100  according to aspects of the disclosure. Power generation plant  100  includes one or more airborne elements or airborne power generating craft, which in one aspect of the disclosure comprises wings, blades, or kites (collectively identified herein as kites  112 ). Kites  112  may be similar to the stiff or substantially non-flexible blades disclosed in  FIG. 1 , or may be at least partially comprised of a flexible material to provide a structure that is rigid, semi-rigid, or non-rigid. For example, kites  112  may flex under the forces of the wind and may be composed of one or more of a rigid material (for example, metal), a semi-rigid material (e.g., carbon fibers), and a non-rigid material (e.g., fabric).  FIG. 3  discloses an aspect in which each kite  112  may include an aircraft-like fuselage  102  to which a rear stabilizer  104  may be attached. A first end  116   a  of each tether line  116  may be attached to a respective one of kites  112 . For example, as shown in  FIG. 4 , first end  116   a  may be attached to a gimbal  124  or other rotating structure on kite  112 . A quick disconnect mechanism  126  may be disposed at or near first end  116   a  to facilitate rapid disconnection of tether line  116  from kite  112 . The quick disconnect mechanism  126  may be configured to be remotely triggered or operated and/or may be manually operated.  FIG. 5  shows a cross-section of a tether line  116 , which may include a tension element  128  that may be constructed of a material having a high strength-to-weight ratio such as carbon fiber, woven cable made of high-strength, corrosion-resistant steel, or the like. In an aspect, tether line  116  is slightly buoyant or includes buoyant elements to prevent it from sinking to the seafloor when not connected to kite  112 . In an aspect, tension element  128  may be made of a material suitable for both subsea (i.e., underwater) and airborne application or deployment. In another aspect, tension element  128  has an underwater component suitable for continuous submersion in a body of water, and an airborne component suitable for use on or above the body of water. The lengths of the underwater and airborne components of tension element  128  may be respectively determined by estimating the depth of the water in which the kite is to be used, and the intended height of kite  112  in operation. Tension element  128  may be designed to surround one or more electrical conduits, shown in  FIG. 5  as an inter-array transmission and communications umbilical cable  130 . Umbilical cable  130  may permit transmission of electrical current supplied to or generated by kite  112 . Umbilical cable  130  may also transmit control and/or diagnostic signals to and/or from the kite  112 , as will be described further herein. Additionally or alternatively, the tether line may include fiber optic or other control and communication elements in addition to the umbilical cable  130 . One design for a tether line  116  is described in PCT Patent Publication WO2012/012429, the disclosure of which is incorporated by reference herein. A layer of insulation  132  may surround and protect umbilical cable  130  from the surrounding water. 
     A second end  116   b  of tether line  116  may be secured at an anchoring point at or on an underwater floor, such as a lake bed, a river bed, or a seafloor  134 , using an anchor pile  136  or similar means. For example, anchor pile  136  may be drilled and grouted, or as shown in  FIG. 6 , may be a driven pile. Alternatively, a vertical load anchor may be used to secure second end  116   b  of tether line  116 . The anchor pile  136  may be located entirely below the surface of the water  138 , as shown in the Figures, but in shallower water part of the anchor pile may be above the surface of the water. A rotating mechanism or element such as a combined gimbal and swivel  140  may be attached to or integrally formed as part of the top of the anchor pile. Second end  116   b  of tether line  116  may then be attached to the gimbal  140 . Tether line  116 , so attached, is permitted to rotate about axes parallel and perpendicular to the seafloor  134 , to thereby enable kite  112  to freely move relative to the anchor pile  136 . A quick disconnect mechanism  142 , shown schematically in  FIG. 6 , is employed at or near the point of connection between the tether line  116  and gimbal  140  to permit the tether to be disconnected and/or replaced if the tether, gimbal, and/or the anchor pile  136  requires maintenance or replacement, or in the event of failure of operations of all or part of the power generation plant  100 . The quick disconnect mechanism  142  may be configured to be remotely triggered or operated and/or may be manually operated. A spool or winch may be included at the anchor pile to permit the cable to be reeled in if the tether breaks or the kite crashes. The spool or winch may include a cable tensioner element that allows the tether line to be reeled in regardless of the amount of tension on the tether line. 
     Kite  112  is designed to move in a path  118 , shown as an elliptical or circular path in  FIG. 2 , in response to the blowing wind W. As the kite moves along the path  118 , tether line  116  moves through the water in an oscillating or repeating pattern. Propellers  120  mounted on the kite rotate and cause electrical current to be generated using motor/generators  122 , to which the propellers are rotatably mounted. The electrical current so generated is transmitted through umbilical cable  130 . The length of each tether line  116  may be selected to enable kites  112  to capture wind energy at a desired altitude, which may exceed 100 meters, or 200 meters, or 300 meters. Each kite may have a nameplate power generation capacity of more than 20 kilowatts, or more than 100 kilowatts, or more than 500 kilowatts, or more than one megawatt, or more than five megawatts. 
     As illustrated in  FIG. 7 , umbilical cable  130  and insulation  132  may diverge from the tension element  128  at a point of separation  142 , which may be at or close to second end  116   b  of tether line  116 , or which may be at any point along the tether line. The umbilical cables associated with each of the tether lines shown in  FIG. 2  are electrically connected in a preferred configuration to an underwater electrical module  146  either directly or by connection to an array line  148 . The array line  148  transmits electrical current generated by the motor/generators to the underwater electrical module  146 , and transmits communications and control signals between each kite  112  and the underwater electrical module. The underwater electrical module  146  contains the infrastructure necessary for basic voltage transformation, power distribution, breaker switching, power isolation, connecting the umbilical cables  130  to the array line  148 , and/or increasing the size of the array line and/or umbilical cables as desired. The underwater electrical module  146  may also harmonize the voltage from the electrical modules and may interconnect the plurality of alternating current (AC) or direct current (DC) sources. The underwater electrical module  146  may perform a DC to DC conversion, an AC to AC conversion, a DC to AC conversion, or an AC to DC conversion, as required. A local electrical distribution cable  150  provides a path for the electrical current routed to underwater electrical module  146  to be sent to an electrical substation, which according to an aspect of the disclosure is an offshore substation  152 . Alternatively, the umbilical cable  130  and/or the array line  148  may be connected directly to the offshore substation  152  without requiring an underwater electrical module  146 . The offshore substation  152  interconnects and directs the flow of electrical current from one or more underwater electrical modules  146 . The offshore substation  152  may harmonize the voltage from the electrical modules and may interconnect the plurality of alternating current (AC) or direct current (DC) sources. The offshore substation  152  may perform a DC to DC conversion, an AC to AC conversion, a DC to AC conversion, or an AC to DC conversion, as required. The offshore substation  152  may provide a location for or a connection to energy storage  154 , if desired. Such energy storage  154  may employ systems or technologies such as underwater pumped storage hydraulic technology, high-temperature thermal energy storage, a fly-wheel, one or more batteries such as a lithium-ion battery, compressed air storage, or other types of energy storage technologies. The offshore substation  152  may also include the capability for electrical isolation, as will be further described herein. The offshore substation  152  may send power to an onshore substation (not shown) through an export cable  156  for connection into a power grid  158  ( FIG. 8 ). Alternatively or additionally, the offshore substation  152  may send power to power machinery located offshore.  FIG. 8  is a top plan view of a representative layout of a power generation plant, according to disclosed aspects, in the form of a wind farm  160 . The wind farm  160  includes twenty-five kites (indicated by their respective paths  118 ), five groups of umbilical lines  130  or array lines  148 , five underwater electrical modules  146 , five local electrical distribution cables  150 , one offshore substation  152 , and one export cable  156 . Wind farm  160  may have any number of kites as desired, and the electric current produced by kites  112  may be electrically connected to export cable  156  through any combination or arrangement of electrical modules, substations, umbilical cables, and electrical distribution cables. 
     Aspects of the disclosure described above anchor kite  112  to the seafloor, thereby eliminating the heavy and expensive offshore towers, semi-submersible structures, and other permanent structures used in known offshore wind farms. However, in some circumstances it may be desirable to limit the range of motion of the kite with respect to the seafloor.  FIG. 9  illustrates the use of a floating structure from which the kite  112  can rotate. The floating structure may be a tension leg platform, spar, semi-submersible structure, a ship-shaped floating structure, or as shown in  FIG. 9 , a buoy  162 . The buoy  162  may be moored to the seafloor at a single point using tether line. Alternatively, multiple lines may be used to moor the buoy at multiple points on the seafloor. In this aspect, tether line  116  may be divided into an underwater portion  116   c  and an aloft portion  116   d.  Each of the portions may then be optimally designed to meet the tension load requirements and to withstand the conditions of its respective environment. Other types of floating structures or foundational members may be used instead of buoy  162 , it being understood that such floating structures are anticipated to be much smaller than those used to support offshore windmill-type motor/generators. Additionally, buoy  162  may include basic electrical infrastructure in an electrical module  164  that results in further simplifying the structure and function of the underwater electrical module  146 . Buoy  162  may also include electrical isolation capability as part of or separate from the electrical module  164  as will be explained below. The electrical module  164  and/or the electrical isolation capability, if provided separately, may be provided in a modular form factor which allows easy removal, installation, repair, and replacement. The electrical module  164  may include any or all of the communications, electrical isolation, and power conversion means as desired. 
     All of the aspects disclosed herein include a kite  112  tethered to the seafloor, and as such there is no fixed point on which the kite can be landed for maintenance, replacement, or when winds are too low or too high for kite to be effectively operated. Known kite systems ( FIG. 1 ) employ a winch or spool to reduce the length of the tether line during such circumstances, but disclosed aspects use a tether line with a constant length between the kite and the anchor pile  136 . In an aspect, kite  112  may be designed to land on the surface of the water  138  and be serviced by a vessel. According to aspects of the disclosure, kites  112  can be landed and transported on a specially outfitted movable structure, barge or vessel, such as an offshore support vessel  170  as depicted in  FIGS. 2 and 10 . The offshore support vessel is designed to move or be moved temporarily to locations where kites  112  have been installed. The offshore support vessel  170  may be outfitted with padded racks or bridles  172  upon which kites  112  may be transported. The offshore support vessel may also include a mount or perch  174  for landing and/or launching kites  112  without spooling or winching in the tether line, or in other words, the deployed length of the tether line (i.e., the length of the tether line between the anchor pile and the kite) is constant during landing and/or launching operations. The offshore support vessel module may additionally include spare tether lines  116 , which may be wound around spools or drums  176  for storage in or on the offshore support vessel. Kites  112  may be controlled, through tether line  116  or via wireless communication/control systems onboard the offshore support vessel, to land on perch  174  for maintenance, repair or replacement. In such a landing operation, propellers  120  powered by motor/generators  122  may provide the required lift to maneuver the kite to the perch or to a water surface. A spare kite  112   a  could replace the landed kite if necessary. Offshore support vessel  170  could service and otherwise perform maintenance and repair on many kites in this manner, thereby eliminating the need for permanent offshore structures to land the kites for maintenance and repair, and eliminating the need to bring the kites onshore for much of the required maintenance and repair thereon. Such onsite installation, removal, service, maintenance, and repair may result in significant cost savings during commissioning, start-up, long-term operation, etc. 
     Another reason known tethered kites have relied upon permanent support structures is to protect the kite from potentially damaging high winds and from situations in which the wind speed is too low to either hold the kite aloft or to generate an acceptable level of power. According to disclosed aspects shown in  FIG. 11 , kite  112  may be programmed to hover horizontally during times of high winds. Kite  112  is shown as having a significant wing shape, which should provide sufficient lift in a high wind situation to keep the kite airborne. Additionally, rear stabilizer  104  may provide lift as well as stability to kite  112  in this situation. On the other hand, kite  112  may be programmed or controlled to hover vertically during times of low winds, as shown in  FIG. 12 . Propellers  120 , powered by motor/generators  122  (shown in  FIG. 3 ), may provide sufficient lift to maintain kite  112  aloft. Motor/generators  122  may be powered by an external power source or through stored power. Alternatively, kite  112  may be programmed or controlled to land on the surface of the water during periods of low winds, tether failure, or loss of grid power. 
     It is anticipated that the tether line  116  could carry electrical power in the range of thousands of volts AC or DC at energy levels of tens of kilowatts to tens of megawatts. Many scenarios exist where the kite  112  or its respective tether line  116  could come into unwanted electrical conduction with the surrounding water or other structures, craft and the like. Aspects disclosed herein include consideration of such electrical safety issues. For example, sensors may be used to detect parameters associated with the kite  112 , its surroundings, and its associated power system. Such parameters may include electrical parameters, such as voltage, lack of voltage, current, current loss, corona discharge, and current and/or voltage unbalance. Such electrical parameters may be measured at any location of the disclosed system. Other detected parameters may include signals indicating degradation of the tether line, altitude of the kite, tension of the tether line, wind speed, height and/or frequency of waves in the body of water in which the kite is installed, the receiving or loss of a trip command from a remote device, the detection of craft or personnel in or approaching the kite, or the presence or absence of a remote signal. Sensors to detect such parameters may include one or more current sensors, voltage sensors, tension monitoring devices, strain gauges, wind meters, communication units, gyroscopes, altimeters, speed sensors, vibration sensors, camera systems, radar, and the like. The detected parameters may be used to determine whether the kite  112  and associated power systems should be switched to a failsafe operating mode or electrical safe state, which in an aspect may be termed a “safe park condition.” The safe park condition may include an electrically safe state or condition. This safe park condition is one which may include de-energizing the tether line  116 . De-energizing the tether may include tripping electrical circuit breakers or activating electrical interrupting devices, and/or turning off the triggering to power electronics devices, which may include gated power electronics such as thyristors and the like. Transition to the safe park condition may include ending power transmission from the kite  112  into the tether line  116  by ending or interrupting electrical conduction to the tether line  116  from the generating source or sources located on the kite, and vice versa. 
     The safe park condition may include ending electrical conduction from the offshore power system by interrupting the electrical connection at any point between offshore substation  152  and kite  112 . The safe park condition may also include grounding the umbilical cable  130  associated with tether line  116 . To facilitate transfer to a safe park condition, electrical switching, interrupting or isolating means should be in electrical communication (preferably in series) with both the first end  116   a  and the second end  116   b  of the tether line  116 . The electrical switching, interrupting or isolating means may be in the form of circuit breakers, pyrotechnic interrupters, switches, power circuit electronics, fuses, grounding switches, and the like. 
     The decision to transition to an electrical safe state, such as the safe park condition, may be incorporated in to the normal operational steps of the kite  112 . For example, if a winged kite  112  were to execute a landing on an offshore support vessel  170 , a transition to the safe park condition may be included as one of the manual or automatically initiated steps of its control system. By way of example, a kite  112  using power from an offshore power system may be programmed or otherwise instructed to operate the motor/generators  122  in a motoring mode (used, e.g., to descend the kite to an offshore supply vessel  170  or to hover the kite during a low wind condition). In such a circumstance, the transition to a safe park condition may be initiated to electrically isolate the tether line from electrical conduction from both the kite and the offshore power system. 
     According to disclosed aspects, electrical switching, interrupting or isolating means may be located at the buoy  162  (if used), in the underwater electrical module  146  as shown by reference number  146   a,  at the offshore substation  152  (if used) as shown by reference number  152   a,  on or in tether line  116  as shown by reference number  117 , or elsewhere in power generation system  100 . Transitioning to the safe park condition may include operating (e.g. opening) the electrical switching, interrupting or isolating means upon receipt of a command from a supervisory control system or via a manual command.  FIG. 13  is a schematic of a representative control system  200  that may be used to initiate a safe park condition or other failsafe mode. Control system  200  may reside on the kite  112 , but may advantageously reside on both the kite and a location not on the kite, such as the buoy  162 , underwater electrical module  146 , and/or offshore substation  152 . Control system  200  may be incorporated into the control system (not shown) used to control flight and autonomous operation of the kite, or alternatively may be independent from other functions. Control system  200  may include a programmable controller  202 , such as an electrically protective relay or a programmable logic controller, which receives input from various sensors  204  as have been previously described. Decision logic may be input at  206  into controller  202  according to known programming principles. Instructions to transition to an electrical safe state, such as the described safe park condition, are output at  208  to the buoy  162 , underwater electrical module  146 , and/or the offshore substation  152  as required. Such output instructions communicate the trigger to the safe park condition when the predetermined requirements for such trigger or transition are sensed, determined, or otherwise requested. 
     An example of a situation in which an electrical failsafe mode may be helpful is if the tether line  116  breaks while the kite  112  is generating power. Sensors  204 , such as current and voltage sensors on the kite, power monitor calculations in the control system of the kite  112 , and/or tension monitors associated with the tether  116  itself, may provide inputs to the programmable controller  202  of the control system  200 . The programmable controller  202  processes the input(s) using decision logic  206  to determine that an abnormal condition has occurred, and will then communicate through outputs  208  to initiating the safe park condition. The tether  116  can thus be safely electrically isolated. 
     In an aspect, conditions requiring electrical isolation are sensed, detected or calculated prior to when an abnormality is detected. It may be desirable for electrical isolation to occur before any abnormal current flow or voltage variation is detected. According to one aspect, the system may anticipate that current carrying conductors or components are approaching an increased risk of electrical fault (e.g., impact with the surface of a body of water). By way of example, sensing an undesirable condition may include sensing a position or calculating the trajectory of the kite or the tether line, and electrical isolation may be performed automatically in response to the anticipated trajectory or position of the kite, prior to an electrical anomaly being detected by sensors  204 . 
     The disclosed aspects have many advantages when compared with known wind energy solutions. Such advantages include significant weight reduction, manufacturing and installation cost, ability to harness wind energy at high altitudes, and the ability to harness wind energy inexpensively at extreme water depths. As such, aspects of the disclosure may be used to not only supply power to a power grid, but may also be used to power any type of offshore project, such as aquaculture or desalination. As another example, aspects of the disclosure may be used to access new oil and/or gas reservoirs adjacent existing an offshore oil and gas facility. If the most cost-effective way to develop the new reservoirs is to leverage the existing infrastructure, there will likely be additional power requirements for such development, especially if the development has significant subsea components. Since the original offshore oil and gas facility likely was not designed with the additional power requirements in mind, it may be expensive and time-consuming to meet the additional power requirements. The disclosed aspects enable additional power generating capacity to be added to the existing offshore facility at a reasonable cost. 
     Aspects of the disclosure may also advantageously be used with new offshore oil and gas projects that require power generation to operate. An offshore platform or facility may be economically powered at least in part by one or more kites as disclosed herein. Such kite-based power is especially attractive for subsea production that leverages existing processing, storage and/or transportation facilities that are a long way (&gt;50 km) from existing subsea production and/or processing infrastructure. 
     Aspects described herein may have other advantageous applications. For example, the disclosed aspects may be used with other power sources, including other renewable sources such as solar, tidal, thermal, geothermal, and the like, to power equipment used in subsea boosting or to be used when one of the renewable sources is not available because of low winds, low available solar energy, grid loss, etc. 
     The disclosed aspects have described a tether line secured at one end to a seafloor and at the other end to a kite. It is to be understood that such a tether line may actually be two separate lines—for example, an underwater portion and an aloft portion—that function together to secure the kite to the seafloor and transmit power generated by movement of the kite to the electrical transmission system. While the two separate lines may have different lengths, diameters, and compositions, for the purposes of this disclosure such separate tether lines or tether line portions may be considered to be a single tether line. 
       FIG. 14  depicts another aspect of the disclosure in which a motor/generator  220  is located at the buoy  162  instead of at the kite. A spool  222  is rotatably connected to motor/generator  220 . Aloft portion  116   d  of the tether line is configured to be wound and unwound around spool  222 . When motor/generator  220  acts as a motor, aloft portion  116   d  of the tether line winds around spool  222 . When spool  222  is directed to unwind the aloft portion of the tether line, the motor/generator  220  generates power that is transmitted through umbilical cable  116   b  to the electrical transmission system (not shown). 
     Because the kite  112  is light and capable of creating aerodynamic lift, it is much easier to transport and install.  FIG. 15  is a schematic illustration of how kite  112  may be transported to or from an installation site. As shown in  FIG. 15 , kite  112  may be attached to a tow cable  230  that is at least partially wound around a spool  232 . In this disclosed aspect, the spool  232  is mounted on a small vessel or boat  234 . Using tow cable  230 , small boat  234  may tow the kite  112  from land or from an offshore support vessel to an installation site  236 , which is typically at a wind farm or other power generation site. Kite  112  may be maintained aloft using motor/generator  122  and the propellers  120 , principles of aerodynamic lift, or both. When the small boat  234  reaches the installation site  236 , tow cable  230  is reeled in until the kite is close enough to secure first end  116   a  of tether line  116  to the kite. The kite may then ascend into the air to generate power as previously described. This procedure may be reversed if a kite is to be removed from an installation site to a land-based landing site, an offshore supply vessel, or other location. The method of transportation and installation/de-installation depicted in  FIG. 15  and described herein is an alternative to using a much larger offshore supply vessel  170 . Alternatively, as described above, an offshore supply vessel may serve primarily to transport kites  112  to and from the general vicinity of their respective installation sites, and one or more small boats  234  may transport kites  112  to and from the offshore supply vessel to install the kites at their respective installation sites. 
       FIG. 16  is a flowchart of a method  300  of generating power according to disclosed aspects. At block  302  an airborne power generating craft is connected to an anchor using a tether line. The anchor is secured to an underwater floor. At block  304  power is generated based on movement of the airborne power generating craft in response to a wind force. At block  306  a constant length of the tether line is maintained between the airborne power generating craft and the anchor as the airborne power generating craft moves in response to the wind force. At block  308  the airborne power generating craft is connected to an electrical transmission system through at least part of the tether line. At block  310  the generated power is transmitted to the electrical transmission system. 
       FIG. 17  is a flowchart of a method  400  of generating power according to disclosed aspects. At block  402  an airborne power generating craft is connected to a floating structure, such as a buoy, using an aloft portion of a tether line. At block  404  the floating structure is connected to an anchor using an underwater portion of the tether line. The anchor is secured to an underwater floor. At block  406  power is generated based on movement of the airborne power generating craft in response to a wind force. At block  408  the floating structure is connected to an electrical transmission system through at least part of the tether line. At block  410  the generated power is transmitted to the electrical transmission system. 
       FIG. 18  is a flowchart of a method  500  of maintaining an offshore power plant according to disclosed aspects. At block  502  a plurality of airborne power generating craft are landed on or near a floating vessel. Each of the plurality of airborne power generating craft forms part of the offshore power plant. 
       FIG. 19  is a flowchart of a method  600  for maintaining an offshore power plant according to disclosed aspects. The offshore power plant has a first airborne power generating craft and a second airborne power generating craft. At block  602  the floating vessel is moved to a position adjacent the first airborne power generating craft. At block  604  the first airborne power generating craft is landed on or near the floating vessel. At block  606  the floating vessel is moved to a location adjacent the second airborne power generating craft. At block  608  the second airborne power generating craft is landed on or near the floating vessel. 
       FIG. 20  is a flowchart of a method  700  for generating power according to disclosed aspects. At block  702  an airborne power generating craft is connected to an anchor using a tether line. The anchor is secured to an underwater floor. At block  704  power is generated based on movement of the airborne power generating craft in response to a wind force. At block  706  a constant length of the tether line is maintained between the airborne power generating craft and the anchor as the airborne power generating craft moves in response to the wind force. At block  708  the airborne power generating craft is connected to an electrical transmission system through at least part of the tether line. At block  710  the generated power is transmitted to the electrical transmission system. At block  712  a condition is sensed in which transmitting power to the electrical transmission system is not desired. At block  714  the airborne power generating craft is electrically isolated to prevent power from being transmitted from the airborne power generating craft to the electrical transmission system. 
       FIG. 21  is a flowchart of a method  800  of maintaining an offshore power plant according to disclosed aspects. At block  802  a power generating craft is attached to a tow cable on a floating vessel. At block  804  the floating vessel is moved to an offshore power generating site. At block  806  the power generating craft is maintained in an airborne state while the floating vessel is moving to the offshore power generating site. At block  808  the power generating craft is detached from the tow cable and attached to a first end of a tether line at the offshore power generating site. A second end of the tether line is anchored to an underwater floor. At block  810  operating the power generating craft is operated in an airborne state. 
       FIG. 22  is a flowchart of a method  900  of maintaining an offshore power plant according to disclosed aspects. At block  902  detaching a power generating craft from a first end of a tether line at an offshore power generating site. A second end of the tether line is anchored to an underwater floor. At block  904  the power generating craft is attached to a tow cable on a floating vessel. At block  906  the floating vessel is moved away from the offshore power generating site. At block  908  the power generating craft is maintained in an airborne state while the floating vessel is moving away from the offshore power generating site. 
     Disclosed aspects may include any combinations of the methods and systems shown in the following numbered paragraphs. This is not to be considered a complete listing of all possible aspects, as any number of variations can be envisioned from the description above.
     D1. A method of generating power, comprising:   

     connecting an airborne power generating craft to an anchor using a tether line, the anchor being secured to an underwater floor; 
     generating power based on movement of the airborne power generating craft in response to a wind force; 
     maintaining a constant length of the tether line between the airborne power generating craft and the anchor as the airborne power generating craft moves in response to the wind force; 
     connecting the airborne power generating craft to an electrical transmission system through at least part of the tether line; 
     transmitting the generated power to the electrical transmission system; 
     sensing a condition in which transmitting power to the electrical transmission system is not desired; and 
     electrically isolating the airborne power generating craft to prevent power from being transmitted from the airborne power generating craft to the electrical transmission system.
     D2. The method of paragraph D1, wherein power is generated using a motor/generator and a propeller installed on the airborne power generating craft, and wherein the condition is a take-off or landing of the airborne power generating craft on a water surface or a floating vessel, the method further comprising: controlling the motor/generator and the propeller to land the power generating craft on a water surface or on a floating vessel.   D3. The method of claim D1, wherein power is generated using a motor/generator and a propeller installed on the airborne power generating craft, and wherein the condition is a wind speed above or below a power generation wind speed range, the method further comprising:   

     controlling the motor/generator and the propeller to hover the airborne power generating craft in a state in which power is not generated by the motor/generator and the propeller.
     D4. The method of any of paragraphs D1-D3, wherein electrically isolating the airborne power generating craft comprises interrupting an electrical current flow at one end of the tether line.   D5. The method of any of paragraphs D1-D4, wherein electrically isolating the airborne power generating craft comprises interrupting an electrical current flow along a length of the tether line.   D6. The method of any of paragraphs D1-D5, wherein electrically isolating the airborne power generating craft comprises interrupting an electrical current flow within the electrical transmission system.   D7. The method of any of paragraphs D1-D6, wherein the tether line is supported by a floating structure, and wherein electrically isolating the airborne power generating craft comprises interrupting an electrical current flow at the floating structure.   D8. The method of paragraph D7, wherein the floating structure is a buoy.   D9. The method of any of paragraphs D1-D8, wherein sensing the condition comprises sensing one or more of degradation of the tether line, tension of part or all of the tether line, altitude of the airborne power generating craft, wind speed, height of waves on a water surface, frequency of waves on a water surface, loss of a command signal, reception of a tripping signal, detecting approaching craft or personnel, and the presence or absence of a remote signal.   D10. The method of any of paragraphs D1-D9, wherein sensing the condition comprises sensing a position or calculating a trajectory of the airborne power generating craft or of the tether line.   D11. The method of paragraph D10, wherein electrically isolating the airborne power generating craft is performed in response to the sensed position or calculated trajectory, and is performed prior to an electrical anomaly being sensed.   D12. The method of any of paragraphs D1-D11, wherein sensing the condition is accomplished using one or more of a tension monitoring device, a strain gauge, a wind meter, a gyroscope, an altimeter, a speed sensor, a vibration sensor, a camera system, and a radar system.   D13. The method of any of paragraphs D1-D12, wherein sensing the condition comprises sensing one or more electrical parameters in one or more of the airborne power generating craft, the tether line, and the electrical transmission system.   D14. The method of paragraph D13, wherein the electrical parameter is one of a current level and a voltage level.   D15. The method of any of paragraphs D1-D14, wherein electrically isolating the airborne power generating craft is accomplished using one or more of a circuit breaker, a pyrotechnic interrupter, a switch, a fuse, and a grounding switch.   D16. An offshore power generation system, comprising:   

     an airborne power generating craft; 
     a tether line connected at a first end to the airborne power generating craft, the tether line having a length; 
     an anchor to which a second end of the tether line is attached, the anchor being secured to an underwater floor, wherein the length of the tether line between the airborne power generating craft and the anchor is constant; 
     an electrical transmission system connected to the airborne power generating craft through the tether line, the electrical transmission system being configured to transmit power generated by the airborne power generating craft; 
     a sensor that senses a condition in which transmitting power to the electrical transmission system is not desired; and 
     an electrical isolation mechanism that prevents power from being transmitted from the airborne power generating craft to the electrical transmission system in response to the sensed condition.
     D17. The offshore power generation system of paragraph D16, wherein the electrical isolation mechanism is one of a circuit breaker, a pyrotechnic interrupter, a switch, a fuse, and a grounding switch.   D18. The offshore power generation system of paragraph D16 or D17, wherein the airborne power generating craft comprises a motor/generator electrically connected to the electrical transmission system through the tether line, wherein the electrical isolation mechanism is positioned to prevent power from being transmitted from the motor/generator to the tether line.   D19. The offshore power generation system of any of paragraphs D16-D18, wherein the electrical isolation mechanism is positioned at an end of the tether line.   D20. The offshore power generation system of any of paragraphs D16-D19, wherein the electrical isolation mechanism is positioned along the tether line.   D21. The offshore power generation system of any of paragraphs D16-D20, wherein the sensor comprises one or more of a tension monitoring device, a strain gauge, a wind meter, a gyroscope, an altimeter, a speed sensor, a vibration sensor, a camera system, and a radar system.   D22. The offshore power generation system of any of paragraphs D16-D21, further comprising a floating structure connected to the tether line, wherein the electrical isolation mechanism is mounted on the floating structure.   D23. The offshore power generation system of paragraph D22, wherein the floating structure is a buoy.   D24. The offshore power generation system of any of paragraphs D16-D23, wherein the tether line includes a tension element configured to secure the airborne power generating craft to the anchor, and an electrically conductive umbilical cable configured to transmit at least one of power and control signals between the airborne power generating craft and the electrical transmission system; and   

     wherein the electrical isolation mechanism is located along the umbilical cable.
     D25. The offshore power generation system of any of paragraphs D16-D24, wherein the electrical transmission system comprises:   

     an underwater electrical module connected to the tether line, the underwater electrical module performing at least one of voltage transformation, power distribution, breaker switching, communication, control, and power isolation; and 
     an offshore substation electrically connected to the underwater electrical module, the offshore substation performing at least one of voltage harmonization, direct current (DC) to DC conversion, DC to alternating current (AC) conversion, AC to DC conversion, and AC to AC conversion; 
     wherein the electrical isolation mechanism is located at one of the underwater electrical module and the offshore substation. 
     It should be understood that the numerous changes, modifications, and alternatives to the preceding disclosure can be made without departing from the scope of the disclosure. The preceding description, therefore, is not meant to limit the scope of the disclosure. Rather, the scope of the disclosure is to be determined only by the appended claims and their equivalents. It is also contemplated that structures and features in the present examples can be altered, rearranged, substituted, deleted, duplicated, combined, or added to each other.