GB2617686A - Mooring systems and methods for floating offshore wind turbines - Google Patents

Abstract

A constant tension mooring system includes at least an anchor pile 18, a plurality of mooring lines 12, fairleads 10, padeyes (36, Fig 10A) and pendant gravity devices 14. The mooring line runs vertically upward from one end attached to the pendant gravity device suspended in mid-water through a fairlead, connected to a floater 4 and extends downward to a second end attached to the anchor pile. The tension force in the mooring line equals to the submerged weight of the pendant gravity device. A high anchor mooring system comprises at least one high anchor post 16 coupled to an anchor pile, a plurality of mooring lines, tri-plates (66, Fig 18), padeyes and pendant gravity devices. The mooring line has one end connected to the floater and a second end to the top of the high anchor post with the tri-plate in the middle connecting the pendant gravity device suspended below in mid-water. The high anchor post extends upward from the seabed to a height forming a V-shaped mooring line configuration.

Description

MOORING SYSTEMS AND METHODS FOR FLOATING OFFSHORE WIND TURBINES

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application claims the benefits of priority from the United States of America nonprovisional application US 17/681, 744, entitled "MOORING SYSTEMS AND METHODS FOR FLOATING OFFSHORE WIND TURBINES", filed on February 26, 2022.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT I0

Not applicable.

REFERENCES

I5 U.S. PATENT DOCUMENTS 7,8 19,073 B2, Sveen et al., October 2010 8,689,721 B2, Wang, April 2014 8,692,401 B2, Roddier et al.. April 2014 9,139,266 B2_ Roddier et al.. September 2015 9,394,035 B2. Dagher et al.. July 2016

OTHER PUBLICATIONS

International Renewable Energy Agency (IRENA), "Floating Foundations: a Game Changer for Offshore Wind Power", 2016.

Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE) "Offshore Wind Market Report -2021 Edition", August 2021.

Journal of Ocean Engineering, Elsevier, "Design and comparative analysis of alternative mooring systems for floating wind turbines in shallow water with emphasis on ultimate limit state design", Volume 219, January 2021.

FIELD OF INVENTION

Embodiments of present invention relate generally to the field of floating offshore wind turbines for wind power generation. More specifically, embodiments of present invention relate to mooring systems and methods for station-keeping of a floating offshore wind turbine.

BACKGROUND OF THE INVENTION

Offshore wind power has become a main source of renewable energy. Presently, most of the offshore wind power is generated using wind turbines with wind power generation capacity less than 5MW supported by fixed bottom foundations in water depth less than 50 meters. As the demand for offshore wind power increases, there are growing needs to deploy large wind turbines supported by floating foundations in water depth greater than 50 meters where conventional fixed bottom foundations become less attractive economically compared with floating foundations for large wind turbines with wind power generation capacity greater than 10MW. In recent years, several novel concepts of floating offshore wind turbine foundations have been invented. Examples of such inventions are U.S. Pat. No. 7,819,073B2 (2010), U.S. Pat. No. 8,689,721B2 (2014), U.S. Pat. No. 8,692,401B2 (2014), U.S. Pat. No. 9,139,266B2 (2015), and U.S. Pat. No. 9,394,03582 (2016). Some of these novel concepts have been or are being deployed to offshore wind farms for commercial power generation in water depth ranging from 50 to 200 meters according to industry and government publications.

Mooring systems are used for station-keeping of a floater such as a floating offshore wind turbine in an acceptable range within its initial equilibrium position. A mooring system generally consists of several mooring lines spreading radially outward in a symmetrical fashion from the floater to the anchors on the seabed. In contrast to the technological innovations and advancement in floating offshore wind turbine foundations, mooring system technologies for station-keeping of floating offshore structures have not advanced significantly in many decades. Presently, the catenary mooring system is the most widely used station-keeping system in the offshore industry for floating oil and gas platforms as well as floating offshore wind turbine foundations. The catenary configuration of the mooring line provides the horizontal restraining force through the weight of the steel chain and wire suspended in the water as the floater having a horizontal offset from its initial equilibritan position.

Traditionally, water depth above 305 meter (1000 feet) is considered as deep water and 150 meters or below as shallow water for offshore oil and gas production in the offshore industry. It is known that the prior art conventional catenary mooring systems have significant shortcomings and technical challenges for floating offshore wind turbines in shallow water, and the problem gets worse as the water depth gets shallower and the suspended catenary section of the mooring line gets shorter. Specifically, for a catenary mooring system to restrain a floating offshore wind turbine in an acceptable range within its initial equilibrium position in shallow water, the mooring line length required usually exceeds 10 times of the water depth resulting in long and costly mooring lines. For example, the mooring line length of a catenary mooring system could exceed 800 meters for a floating offshore wind turbine in a water depth of 50 meters. FIG. lA illustrates a front view of an exemplary floating offshore wind turbine with a catenary mooring system in approximately 50m water depth according to the prior art, with the mooring line length on the seabed truncated for clarity. FIG. 1B illustrates the same front view of the floating offshore wind turbine shown in FIG. lA with the catenary mooring line drawn to scale on the right side showing a very long mooring line on the seabed. FIG. 2A illustrates a top view of the floating offshore wind turbine shown in FTG. 1A with a catenary mooring system consisting of three mooring lines and three anchors with the mooring line length truncated for clarity. FTG. 2B illustrates the same top view of the floating offshore wind turbine shown in FIG. 2A with the catenary mooring system drawn to scale showing a very large mooring footprint on the seabed.

The existing catenary mooring systems according to the prior art are length constraint, i.e., the physical length of the mooring line between the first end at the fairlead on the floater and the second end at the anchor on the seabed is fixed even though the mooring line configuration may vary with water depth and pre-tension. For a length constraint mooring system, the tension force in the mooring line increases as the horizontal offset of the floater increases due to ocean environmental forces from wind, wave and current. For a typical floating offshore wind turbine moored in the ocean using the conventional catenary mooring system, the tension force in the mooring line is dynamic and position dependent. The mooring line tension force can increase exponentially from 1,000KN at the initial position to well over 10,000KN as the horizontal offset of the floater increases, particularly near the maximum offset when the mooring line is becoming a nearly straight line from the initial catenary configuration. This dynamic effect gets worse in shallow water due to the limited water column for the suspended catenary part of the mooring line to accommodate the relatively large horizontal offset of the floater. It is this exponentially increasing tension force that causes significant technical challenges in catenary mooring system design with high risks of potential strength and fatigue failure. Because of the highly dynamic nature of the mooring line tension force, the mooring line size for the conventional catenary mooring system is usually quite large, and often redundant lines are required for safety considerations resulting in increased cost.

The shortcomings of the convention& catenary mooring systems make large scale deployment of floating offshore wind turbines less economical and technically challenging, particularly for shallow water. In addition, the large mooring footprint on the seabed is not friendly to the ocean environment as it causes hazards to marine wildlife and fishery. There remains a need for new mooring systems and methods to improve the existing mooring systems and reduce cost.

SUMMARY OF THE INVENTION

The present disclosure provides mooring systems and methods for station-keeping of floating offshore wind turbines with a much smaller mooring footprint on the seabed, significantly lower cost, and improved safety in comparison with existing conventional catenary mooring systems. Two types of mooring systems are disclosed.

The first type is a constant tension mooring system comprising at least three mooring lines mid the tension force in each of the mooring lines remain constant. In some embodiments as shown in FIG. 3, the constant tension mooring system has a single anchor configuration comprising an anchor head structure coupled to an anchor pile fixed to a seabed; a floating body having an upper hull supporting the wind turbine and a lower hull connecting the mooring system; at least three mooring lines, fairleads, padeyes, pendent gravity devices and fairlead supports. Each of the mooring lines has one end attached to the corresponding pendent gravity device and runs vertically upward through the corresponding fairlead then extending radially downward to a second end attached to the anchor head structure located in a center of a mooring pattern which is an intersection of a vertical centerline of the floating body and the seabed. Each of the pendent gravity device is suspended in a mid-water below the fairlead attached to the lower hull causing a tension force in the mooring line equal to the submerged weight of the pendent gravity device. Specifically, the fairlead has a wheel with a grooved rim around which the mooring line runs through from one side to another side acting to change the direction of the tension force applied to the mooring line while maintaining a constant magnitude of the tension force. The magnitude of the tension force of the mooring line on both sides of the wheel equals to the submerged weight of the pendent gravity device as shown in FIG. SA. In contrast to the prior art catenary mooring systems with fixed mooring line length, the mooring line length between the fairlead and the anchor pile varies when the floating body has a horizontal offset from its initial position according to the present invention as shown in FIG. 6.

In some other embodiments, the constant tension mooring system has an anchor extension structure coupled to the top of the anchor pile in a relatively deeper water. The fairleads are attached to a fairlead support coupled to a top of the anchor extension structure which extends vertically upward from the seabed to a substantial height below the lower hull. Each of the mooring lines has one end attached to the pendent gravity device suspended in a mid-water running vertically upward through the corresponding fairlead then extending radially upward to a second end connected to the corresponding padeye attached to an outer perimeter of the lower hull. The anchor extension structure has an adequate height to allow the pendent gravity device to move up or down when the floating body has a horizontal offset. In this case, as shown in FIG. 11 the submerged weights of the pendent gravity devices are supported by the anchor extension structure and the tension forces in the mooring lines cause a compressive force for the anchor pile. This is different from the constant tension mooring system shown in FIG. 3 which has the submerged weights of the pendent gravity devices supported by the lower hull and the tension forces in the mooring lines cause an uplift force for the anchor pile.

As shown in FIG 14, another embodiment of the constant tension mooring system has a configuration with multiple anchor piles comprising at least three anchor piles, fairleads, mooring lines, pendent gravity devices and anchor head structures each coupled to a top of the corresponding anchor pile fixed to the seabed. Each of the mooring lines has one end attached to the corresponding pendent gravity device suspended in a mid-water running vertically upward through the corresponding fairlead attached to the lower hull then extending radially outward to a second end attached to the corresponding anchor head structure. Each of the pendent gravity device is initially suspended in mid-water at a depth approximately halfway between the seabed and the fairlead. The anchor pile has a horizontal distance to the corresponding fairlead normally less or equal to the water depth.

The second type is a high anchor mooring system comprising at least one high anchor post coupled to an anchor pile fixed to the seabed. In some embodiments, the high anchor mooring system has a single anchor configuration with a high anchor post coupled to an anchor pile comprising at least three mooring lines, hull padeyes, anchor padeyes, tri-plates, and pendent gravity devices. Each of the mooring lines has two segments linked by the tri-plate connecting the corresponding hull padeye at one end and the corresponding anchor padeye coupled to a top of the corresponding high anchor post at a second end. The pendent gravity device is suspended in a mid-water below the tri-plate linked via a short chain or rope. The high anchor post is in the center of the mooring pattern anchoring each of the mooring lines and extends vertically upward from the seabed to a height so that both ends of the mooring line are higher in elevation than the tri-plate forming a V-shaped mooring line configuration providing an adequate total length for maximum horizontal offset of the floater as shown in FIGS. 16 and 17. The pendent gravity device moves up and down as the floater moves away from As initial position as shown in FIG. 19. In some other embodiments, the present disclosure has multiple anchor piles with corresponding high anchor posts as shown in FIGS. 22, 23 and 25. Each of the anchor piles are located radially outward from the center of the mooring pattern with a horizontal distance to the corresponding hull padeye normally less or equal to the water depth.

A Method for station-keeping of a floating body is disclosed, comprising: providing a mooring system with an anchor pile fixed to a seabed, a plurality of mooring lines, fairleads and pendent gravity devices forming a mooring pattern; attaching the fairleads to the floating body; setting the anchor pile at the center of the mooring pattern; connecting one end of the mooring line to the pendent gravity device; running the mooring line vertically upward through the fairlead; connecting the other end of the mooring line to the anchor pile; generating a tension force of the mooring line by suspending the pendent gravity device in a mid-water below the fairlead; having a horizontal offset of the floating body; causing a first set of the mooring lines having the mooring line length to increase and the corresponding pendent gravity devices to move up for, and a second set of mooring lines in the opposite direction having the mooring line length to decrease and the corresponding pendent gravity devices to move down; causing a greater horizontal component force for the first set of mooring lines and a smaller horizontal component force for the second set of mooring lines; proving a horizontal restoring force for station-keeping of the floating body.

BRIEF DESCRIPTION OF DRAWINGS

FIG. IA is a schematic front view of an exemplary floating offshore wind turbine in approximately 50m water depth having a conventional catenary mooring system according to the prior art with mooring line length truncated for clarity.

FIG. 1B is a schematic front view of the floating offshore wind turbine shown in FIG. lA with mooring line length drawn to scale on the right side.

FIG. 2A is a schematic top view of the floating offshore wind turbine shown in FIG. 1A having three mooring lines and three anchors with mooring line length truncated for clarity.

FIG. 2B is a schematic top view of the floating offshore wind turbine shown in FIG. 2A with mooring line length drawn to scale.

FIG. 3 is a schematic front view of an exemplary embodiment of a floating offshore wind turbine with a constant tension mooring system having an anchor pile in accordance with the present invention.

FIG. 4 is a schematic top view of the floating offshore wind turbine with an embodiment of the constant tension mooring system shown in FTG. 3 having three mooring lines.

FIG. SA is a schematic detailed view of an embodiment of a mooring line running through a fairlead with a pendent gravity device suspended below.

FIG. SB is a schematic detailed view of another embodiment of the pendent gravity device.

FIG. 6 is a schematic front view of an embodiment of the floating offshore wind turbine with the constant tension mooring system shown in FIG. 3 at a maximum horizontal offset.

FIG. 7 is a graphic comparison of mooring line tension force vs. horizontal offset between the prior art conventional catenary mooring system and the constant tension mooring system.

FIG. 8 is a graphic comparison of mooring system restoring force vs. horizontal offset between the prior art conventional catenary mooring system and the constant tension mooring system.

FIG. 9 is a schematic front view of another embodiment of the floating offshore wind turbine with the constant tension mooring system shown in FTG. 3 in a relatively deeper water depth.

FIG. 10A is a detailed prospective view of an embodiment of the anchor pile with three mooring padeyes of the constant tension mooring system.

FIG. 10B is a detailed top view of the mooring padeyes shown in FIG. 10A.

FIG. 11 is a schematic front view of the floating offshore wind turbine with another embodiment of a constant tension mooring system having an anchor pile and an anchor extension structure.

FIG. 12 is a schematic top view of the constant tension mooring system shown in FIG. 11 having three mooring lines.

FIG. 13 is a detailed prospective view of an embodiment of a mooring fairlead attached to a top of the anchor extension structure with a pendent gravity device suspended below.

FIG. 14 is a schematic front view of the floating offshore wind turbine with an embodiment of a constant tension mooring system having multiple anchor piles.

FIG. 15 is a schematic top view of the floating offshore wind turbine with the constant tension mooring system shown in FIG. 14 having three mooring lines and three anchor piles.

FIG. 16 is a schematic front view of an exemplary embodiment of a floating offshore wind turbine with a high anchor mooring system having an anchor pile in accordance with the present invention. FIG. 17 is a schematic top view of the floating offshore wind turbine with the high anchor mooring system shown in FIG. 16 having three mooring lines.

FIG. 18 is a detailed view of an embodiment of a tri-plate connection in a middle portion of a mooring line with a mid-water pendent gravity device.

FIG. 19 is a schematic front view of an embodiment of the floating offshore wind turbine with the high anchor mooring system shown in FIG. 16 at a maximum horizontal offset.

FIG. 20 is a graphic comparison of mooring line tension force vs. horizontal offset between the prior art conventional catenary mooring system and the high anchor mooring system.

FIG. 21 is a graphic comparison of mooring system horizontal restoring force vs. horizontal offset between the prior art conventional catenary mooring system and the high anchor mooring system. FIG. 22 is a schematic front view of another embodiment of the floating offshore wind turbine with a high anchor mooring system having multiple anchor piles.

FIG. 23 is a schematic top view of the floating offshore wind turbine with the high anchor mooring system having three mooring lines and three anchor piles.

FIG. 24 is a schematic front view of an embodiment of the floating offshore wind turbine with the high anchor mooring system shown in FIG. 22 at the maximum horizontal offset.

FIG. 25 is a schematic front view of another embodiment of the floating offshore wind turbine with the high anchor mooring system shown in FIG. 22 having multiple anchor piles in a relatively deeper water depth.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to mooring systems and methods for station-keeping of a floating offshore wind turbine with a small footprint on the seabed. Before explaining the invention in detail, it is to be understood that the present invention is not limited to the embodiments as disclosed and that it can be practiced or carried out in various ways. It is understood that although the disclosed mooring systems and methods are generally intended for shallow water, they can be used in any body of water not limited by water depth, and with any type of floating bodies not limited to floating offshore wind turbines. In the text, if not specified, the word "offset" means horizontal offset; and "move" means move in horizontal direction. The use of relational terms, such as, but not limited to, "top," "bottom," "left," "right," "upper," "lower," "deeper," "shallower," "up," "down," "side," and the like in the written text is for clarity in specific reference to the Figures and is not intended to limit the scope of the invention or the claims.

Preferred embodiments of the present invention are shown in FIGS. 3 through 25 with detailed description as follows.

With reference to FIG. 3, a constant tension mooring system for station-keeping of a floating offshore wind turbine with an anchor pile 18, comprises a floating body having an upper hull 2 extending vertically above a waterline 101 and a lower hull 4 coupled to the upper hull 2 below the waterline 101; a wind turbine rotor assembly; 6 coupled to a top of a turbine tower 8 supported by the upper hull 2; a plurality of fairleads 10, fairlead supports 4A, mooring lines 12 and pendent gravity devices 14 each having a submerged weight; an anchor head structure 16 coupled to a top of the anchor pile 18 fixed to a seabed 201. Each of the fairleads 10 is attached to each of the corresponding fairlead supports 4A coupled symmetrically to an outer perimeter of the lower hull 4 extending a distance radially outward; each of the mooring lines 12 has one end attached to each of the corresponding pcndcnt gravity devices 14 from which running vertically upward through each of the corresponding fairleads 10 then extending radially downward to a second end attached to a top of the anchor head structure 16 in a center of a mooring pattern which is an intersection of a vertical centerline of the floating offshore wind turbine and the seabed 201; each of the pendent gravity devices 14 is suspended in a mid-water below each of the corresponding fairleads 10 causing a tension force in each of the corresponding mooring lines equal to the submerged weight the pendent gravity device.

With reference to FIG. 4, the constant tension mooring system shown in FIG. 3 consists essentially of three fairlead supports 4A, fairleads 10, mooring lines 12 and pendent gravity devices 14 forming a symmetrical mooring pattern; the anchor head structure 16 is coupled to the anchor pile 18 located in a center of the mooring pattern: each of the mooring lines 12 runs radially outward from the anchor head structure 16 to each of the corresponding fairleads 10 attached to each of the corresponding fairlead supports 4A coupled symmetrically to an outer perimeter of the lower hull 4 extending radially outward.

With reference to FIG. SA, the fairlead 10 comprises a wheel with a grooved rim around which the mooring line 12 runs through from one side to another side and acts to change a direction of a tension force applied to the mooring line 12 while maintaining a constant magnitude of the tension force. The magnitude of the tension force of the mooring line 12 on both sides of the wheel equals to the submerged weight of the pendent gravity device 14 suspended vertically in the water below the fairlead 10.

With reference to FIG. 5B, a detailed embodiment of the pendent gravity device 14 consists essentially of a large main gravity unit 14M and a plurality of small secondary gravity units 14S which are of preferably spherical shape suspended below the main gravity unit 14M each linked via a short chain or rope 14L. The main gravity unit 14M preferably has a height of Ito 3m with a cross section of square, circle, rectangle, or other shapes. In some preferred embodiment, the pcndcnt gravity device 14 has only the main gravity unit 14M without the secondary gravity units 14S.

The material of the mooring line 12 may be of steel chain, steel wire or synthetic material such as polyester or nylon. The material of the pendent gravity device 14 may be of steel, iron ore, concrete, or other types of heavy materials. The breaking strength of the mooring line 12 should be determined based on the submerged weight of the pendent gravity device 14 with a safety factor. The initial elevation of the pendent gravity device 14 is normally set at a depth approximately a halfway between the seabed 201 and the fairlead 10 to maximize the up and down traveling range.

With reference to FIG. 6, a schematic front view of the floating offshore wind turbine shown in FIG. 3 with the constant tension mooring system is illustrated at a maximum horizontal offset position. The main function of a mooring system is to restrain the position of a floater by providing a horizontal restoring force as the floater moves away from an initial equilibrium position. For simplicity of this description, the constant tension mooring system is illustrated with two mooring lines 12A and 12B, two fairleads 10A and 10B, two pendent gravity devices 14A and 14B each having a submerged weight and one anchor head structure 16 in a two-dimensional arrangement as shown in FIG. 6. The mooring line 12A has a length between the fairlead 10A and the anchor head structure 16, and this length increases and the corresponding pendent gravity device 14A moves up to an upper limit below the fairlead 10A when the floating offshore wind turbine moves in a right direction to the maximum horizontal offset position from the anchor head structure 16. in the meantime, the mooring line I 2B in an opposite direction of the mooring line I 2A has a length between the fairlead 10B and the anchor head structure 16, and this length decreases and the corresponding pendent gravity device 14B moves down to a lower limit above the seabed 201 as the fairlead 10B moves in the right direction with the floating offshore wind turbine closer to the anchor head structure 16. The mooring line tension forces of the mooring lines 12A and 12B remain constant equal to the submerged weight of the pendent gravity devices 14A and 14B, respectively, due to a pulley effect when the floating offshore wind turbine moves from an initial position as shown in FIG. 3 to the maximum horizontal offset position as shown in FIG. 6. Each of the mooring lines 12A and 12B has a corresponding mooring angle with respect to the waterline 101. The mooring line angle becomes smaller for the mooring line 12A and larger for the mooring line 12B. This gives a larger horizontal component force for the mooring line 12A and smaller horizontal component force for the mooring line 12B than the respective initial horizontal forces at the initial equilibrium position. The mooring system restoring force is a resultant force of the horizontal component forces of the mooring line tension forces in mooring lines I2A and I2B. The initial mooring line angle for each of the mooring lines I 2A and I 2B is normally set between 30 to 60 degrees. For the floating offshore wind turbine in a water with a water depth less than 60 meters, the pendent gravity devices 14A and 14B may be initially set closer to the seabed 201 at a distance equal to approximately 10% to 15% of the water depth to allow more upside travel range of the pendent gravity device 14A and accommodate more horizontal offset as the water depth may not be deep enough for a full up and down travel range of the pendent gravity devices 14A and 14B. The pendent gravity device 14B may touch the seabed and a part of the mooring line 12B below the fairlead 10B may become loose. In this case, a preferred configuration for each of the pendent gravity devices 14A and 14B is in accordance with the detailed embodiment of the pendent gravity device shown in FIG. 5B consisting essentially of a large main gravity unit 14M and a plurality of small secondary gravity units 14S which are of spherical shape suspended below the main gravity unit. In addition, a greater safety factor should be provided for the mooring lines to account for potential shock loads or snap loads when the loose mooring line is suddenly tensioned as the floating offshore wind turbine starts to move back from the maximum horizontal offset and the pendent gravity device is lifted from the seabed.

With reference to FIG. 7, a graphic comparison of a mooring line tension force of the constant tension mooring system to the prior art conventional catenary mooring system vs. a horizontal offset for an exemplary floating offshore wind turbine in a shallow water depth is presented. The constant tension mooring system has a mooring line tension curve 301 which is flat at approximately 5,000kn. In contrast, the conventional catenary mooring system has a mooring line tension curve 302 which is at approximately 2,000kn initially and increases exponentially to approximately 30,0001(n as the horizontal offset increases from 0 to 20m.

With reference to FIG. 8, a graphic comparison of a mooring system restoring force of the constant tension mooring system to the prior art conventional catenary mooring system vs. a horizontal offset for the floating offshore wind turbine mentioned in FIG. 7 is presented. The constant tension mooring system has a restoring force curve 401 which is increasing gradually from 0 to approximately 2,500ku as the horizontal offset increases from 0 to 20m. In contrast, the conventional catenary mooring system has a restoring force curve 402 which is increasing gradually from 0 initially then increases exponentially to nearly 25,000kn as the horizontal offset approaches 20m.

It is noted that the restoring force of the constant tension mooring system is practically the same as the conventional catenary mooring system for a relatively large horizontal offset range from 0 to approximately 15m which would represent a horizontal excursion equal to 30% of a 50m water depth. This indicates that the constant tension mooring system performs the station-keeping function equally well with a much smaller mooring line tension and shorter mooring line length in comparison with the prior art conventional catenary mooring system.

With reference to FIG. 9, the constant tension mooring system having an anchor pile 18 shown in FIG. 3 is deployed in a relatively deeper water depth by increasing a length of the anchor head structure 16 and a mooring line angle for each of the mooring lines 12. A length of the fairlead support 4A may also be increased to maintain a proper geometry for the mooring system. This is illustrated with the following example. Assuming the floating offshore wind turbine has a draft of 20m with the fairlead 10 at the same draft and a horizontal distance from the outer perimeter of the lower hull 4 to a center of the floating offshore wind turbine is 40m, for a 100m water depth with a mooring line angle of 60 degrees and a 5m length for the anchor head structure 16, the horizontal distance from the fairlead 10 to the center would be approximately 43.3m. This will result in approximately a 3.3m horizontal length for the fairlead support 4A which is quite reasonable for practical purposes.

With reference to FIGS. 10A and 10B, the anchor head structure 16 shown in FIG. 3 comprises an upper section 32 and lower section 34 coupled to a top of the anchor pile 18 at the seabed 201; a plurality of mooring padeyes 36 coupled to a top of the upper section 32; a plurality of link plates 38 connecting the padeyes 36. The upper section 32 and lower section 34 may be of tubular steel structure or other type of structures. Specifically, for the constant tension mooring system consisting essentially of three mooring lines as shown in FIG. 5, there will be three mooring padeyes 36 coupled to the top of the upper section 32 symmetrically as shown in FIGS. 10A and 10B.

As illustrated in FIG. 11, another embodiment of the floating offshore wind turbine with a constant tension mooring system having an anchor pile 18, comprises a plurality of padeyes 40, mooring lines 42, pendent gravity devices 44 each having a submerged weight and fairleads 46, a fairlead support structure 48 and an anchor extension structure 20 coupled to a top of the anchor pile 18 fixed to the seabed 201 at a center of a mooring pattern which is an intersection of a vertical centerline of the floating offshore wind turbine mid the seabed 201. Each of the padeyes 40 is attached symmetrically to an outer perimeter of a lower hull 4; each of the fairleads 46 is attached symmetrically to the fairlead support 48 coupled to a top of the anchor extension structure 20 which extends vertically upward from the seabed 201 to a substantial height below the lower hull 4. Each of the mooring lines 42 has one end attached to a corresponding one of the pendent gravity devices 44 suspended in a mid-water above the seabed 201 running vertically upward through a corresponding one of the fairleads 46, then extending radially upward to a second end connected to a corresponding one of the padeyes 40. The anchor extension structure 20 has an adequate height to allow the pendent gravity device 44 to move up or down when the floating offshore wind turbine has a horizontal offset.

As illustrated in FIG. 12, another embodiment of the constant tension mooring system shown in FIG. 11 consists essentially three padeyes 40, mooring lines 42, pendent gravity devices 44 and fairleads 46 forming a symmetrical mooring pattern with the anchor extension structure 20 coupled to the anchor pile 18 located in a center of the mooring pattern. Each of the mooring lines 42 with the first end connected to each of the corresponding pendent gravity device 44 running radially outward through each of the corresponding fairleads 46 symmetrically attached to the fairlead support structure 48 to the second end attached to each of the corresponding padeyes 40 coupled symmetrically to the outer perimeter of the lower hull 4.

As illustrated in FIG. 13, the fairlead 46 attached to the fairlead support 48 comprises a wheel with a grooved rim around which the mooring line 42 runs through from one side to another side acting to change a direction of a tension force applied to the mooring line 42 while maintaining a constant magnitude of the tension force. The magnitude of the tension force of the mooring line 42 on both sides of the wheel equals to the submerged weight of the pendent gravity device 44 suspended vertically in a mid-water below the fairlead 46 It is noted that the constant tension mooring system shown in FIG. 11 has the submerged weights of the pendent gravity devices 44 supported by the anchor extension structure 20 and the tension forces in the mooring lines 42 cause a compressive force for the anchor pile 18. In contrast, the constant tension mooring system shown in FIG. 3 has the submerged weights of the pendent gravity devices 14 supported by the lower hull 4 and the tension forces in the mooring lines 12 cause an uplift force for the anchor pile 18.

With reference to FIG 14, another embodiment of the floating offshore wind turbine with a constant tension mooring system having a plurality of anchor piles 54, comprises a plurality of fairleads 10, mooring lines 12, pendent gravity devices 14, and anchor head structures 52 each coupled to a top of a corresponding anchor pile 54 fixed to the seabed 201. Each of the fairlead 10 is attached to an outer perimeter of the lower hull 4. Each of the mooring lines 12 has one end attached to a corresponding pendent gravity device 14 running vertically upward through a corresponding fairlead 10 then extending radially downward to a second end attached to a top of a corresponding anchor head structure 52. The pendent gravity device 14 is initially suspended in a mid-water at a depth approximately halfway between the seabed 201 and the fairlead 10. The anchor pile 54 has a horizontal distance to the fairlead 10 normally less than or equal to a vertical distance from the waterline 101 to the seabed 201.

With reference to FIG. 15, the constant tension mooring system shown in FTG. 14 consists essentially of three fairleads 10, three mooring lines It three pendent gravity devices 14, three anchor head structures 52 and three anchor piles 54 forming a symmetrical mooring pattern. Each of the mooring lines 12 extends radially outward from the fairlead 10 attached to an outer perimeter of the lower hull 4 to the corresponding anchor head structure 52 coupled to the corresponding anchor pile 54.

It is noted that the constant tension mooring system shown in FTG. 14 having multiple anchor piles can be deployed readily to 200m water depth and beyond while still maintaining a reasonably small mooring footprint on the seabed with the radius of the mooring circle less than the water depth. Specifically, for the sake of this description, assuming that the floating offshore wind turbine has a draft of 20m and each of the mooring lines has a mooring angle initially at 45 degrees in a water depth of 200m, the anchor pile 54 will have a horizontal distance to the fairlead 10 equal to approximately 180m less than the 200m water depth, mid also less than 1/4 of a typical anchor distance of over 800m for the prior art conventional catenary mooring system in the same water depth.

With reference to FIG. 16, a floating offshore wind turbine with a high anchor mooring system having an anchor pile 74, comprises an upper hull 2 extending vertically above a waterline 101; a lower hull 4 coupled to the upper hull 2 below the waterline 101; a wind turbine rotor assembly 6 coupled to a top of a turbine tower 8 supported by the upper hull 2; a plurality of hull padeyes 60, padeye supports 70, mooring lines 62, pendent gravity devices 64 each having a submerged weight. trip-plates 66 and anchor padeyes 68 attached to a top of a high anchor post 72 which is coupled to a top of the anchor pile 74 fixed to the seabed 201. Each of the hull padeyes 60 is attached to a corresponding one of the padeye supports 70 coupled symmetrically to an outer perimeter of the lower hull 4. Each of the mooring lines 62 has two segments linked by a corresponding one of the tri-plates 66 connecting to a corresponding one of the hull padeyes 60 at a first end and a corresponding one of the anchor padeyes 68 at a second end with mooring line angles between the mooring lines 62 and the waterline 101 in a range of 30 to 60 degrees at both ends. Each of the pendent gravity devices 64 is suspended in a mid-water below a corresponding one of the tri-plates 66 linked by a chain or rope above the seabed. The high anchor post 72 extends vertically upward from the seabed 201 to a substantial height so that each of the the mooring lines 62 having both the first and second ends higher in elevation than the corresponding one of the tri-plate 66 forming a V-shaped mooring line configuration providing a total mooring line length for a maximum horizontal offset of the floating offshore wind turbine. This means that the mooring line 62 runs downward from the first end at the corresponding hull padeye 60 to the corresponding tri-plate 66 with the corresponding pendent gravity device 64 suspended below linked by a chain or rope 76, then extends upward to the second end at the corresponding anchor padcyc 68. In one embodiment, the tri-plate 66 is preferably set at a depth approximately halfway between the seabed 201 and the anchor padcyc 68 to allow for up or down movement of the pendent gravity devices 64 when the floating offshore wind turbine has a horizontal offset.

With reference to FIG. 17, the high anchor mooring system shown in FIG. 16 consists essentially of three hull padeyes 60, three mooring lines 62, three pendent gravity devices 64, three trip-plates 66, three anchor padeyes 68 and three padeye supports 70 forming a symmetrical mooring pattern; the high anchor post 72 is located in a center of the mooring pattern with each of the mooring lines 62 extending radially outward from a corresponding one of the anchor padcyes 68 attached symmetrically to the high anchor post 72 coupled to the anchor pile 74 to a corresponding one of the hull padeye 60 attached to a corresponding one of the padeye supports 70 coupled symmetrically on the outer perimeter of the lower hull 4.

It is noted that details of the anchor padeyes 68 at the top of the high anchor post 72 shown in FIGS. 16 and 17 are the same as those shown in FIGS. 10A and 10B.

With reference to FIG. 18, the tri-plate 66 is a triangular shaped plate arranged upside down with two upper corners connecting the mooring line 62 and a lower corner connecting the pendent gravity device 64 below linked with a chain 76. The mooring line 62 has a mooring line tension force 62R on the right of the tri-plate 66 normally different from a mooring line tension force 62L on the left. A sum of vertical component forces of the mooring line tension forces 62R and 62L equals to a tension force 76T in the chain 76 linking the tri-plate 66 and the pendent gravity device 64. For a configuration of the mooring line 62 with given mooring line angles on both sides of the trip-plate 66, the mooring line tension forces 62R and 62L are calculated based on that the tension force 76T equals to the submerged weight of the pendent gravity device 64.

With reference to FIG. 19, a schematic front view of the floating offshore wind turbine shown in FIG. 16 with the high anchor mooring system is illustrated at a maximum horizontal offset. For simplicity of this description, the high anchor mooring system having an anchor pile 74 is illustrated using two sets of mooring lines 62A and 62B with corresponding pendent gravity devices 64A and 64B, tri-plate 66A and 66B, and fairlead 60A and 60B in a two-dimensional arrangement. As the floating offshore wind turbine moves in a right direction to a maximum horizontal offset with respect to the anchor pile 74 fixed to the seabed 201, the tri-plate 66A moves up to an upper limit with the pendent gravity device 64A, and the tri-plate 66B moves down to a lower limit with the pendent gravity device 64B just above the seabed 201. This results in a greater mooring line tension force in the mooring line 62A as the V-shaped mooring line configuration is getting shallower, and a smaller mooring line tension force in the mooring line 62B as the V-shaped mooring line configuration is getting steeper. In the meantime, this also gives a mooring system restoring force which is a resultant force of the horizontal component forces of the mooring line tension forces at the fairlead 60A and 60B in the mooring lines 62A and 62B. The V-shaped mooring line configurations and mooring line angles of the mooring lines 62A and 62B can be obtained for a given horizonal offset based on geometry, and the mooring line tension forces and the mooring system restoring force can then be derived based on the submerged weights of the pendent gravity devices 64A and 64B, respectively.

With reference to FIG. 20, a graphic comparison of the mooring line tension force of the high anchor mooring system to the prior art conventional catenary mooring system vs. horizontal offset for an exemplary floating offshore wind turbine in a shallow water depth is presented. The high anchor mooring system has a mooring line tension force curve 501 which increases gradually from approximately 2000kn to approximate 70001(n. In contrast, the conventional catenary mooring system has a mooring line tension curve 502 which is also equal to approximately 2,0001m initially and increases exponentially to approximately 30,000kn as the horizontal offset increases from 0 to 20m.

With reference to FIG. 21, a graphic comparison of the mooring system restoring force of the high anchor mooring system to the prior art conventional catenary mooring system vs. horizontal offset for the floating offshore wind turbine mentioned in FIG. 20 is presented. The high anchor mooring system has a restoring force curve 601 which is increasing gradually from 0 to approximately 6,000kn as the horizontal offset increases from 0 to 20m. In contrast, the convention& catenary mooring system has a restoring force curve 602 which is increasing gradually from 0 initially then increases exponentially to nearly 25,000kn as the horizontal offset approaches 20m.

With reference to FIG. 22, a schematic front view of another embodiment of the floating offshore wind turbine with a high anchor mooring system is illustrated, having a plurality of anchor piles 84 fixed to the seabed 201, hull padeyes 60, mooring lines 62, pendent gravity devices 64, trip-plates 66, anchor padeyes 68, and high anchor posts 82 forming a symmetrical mooring pattern. Each of the hull padeyes 60 is attached symmetrically to an outer perimeter of the lower hull 4. Each of the mooring lines 62 having one end attached to a corresponding one of the hull padeyes 60, and running downward to a corresponding one of the tri-plates 66, then extending upward to a second end connected to a corresponding one of the anchor padeyes 68 attached to a top of a corresponding one of the high anchor posts 82 coupled to a top of a corresponding one of the anchor piles 84 located at a horizontal distance radially from a center of the mooring pattern which is an intersection of a vertical centerline of the floating offshore wind turbine and the seabed 201. It is seen that the high anchor post 82 extends vertically upward from the seabed 201 to a heigh in a way so that both ends of the mooring line 62 are higher in elevation than the trnplate 66 forming a V-shaped mooring line configuration with mooring angles in a range of 30 to 60 degrees at both ends providing a total mooring line length for a maximum horizontal offset of the floating offshore wind turbine. More specifically, for 50m water depth, the elevation of the anchor padeye 68 at the top of the high anchor post 82 may be set Lo be approximately the same level as the hull padeye 60. The mid-water pendent gravity device 64 moves up and down with the tri-plate 66 as the floating offshore wind turbine having a horizontal offset. The horizontal distance from the anchor pile 84 to the hull fairlead 60 is normally in a range between one half and one of a vertical distance from the waterline 101 to the seabed 201 with the mooring line angle at the fairlead 60 between 30 to 60 degrees. The details of the tri-plate 66 arrangement arc the same as those shown in FIG. 18.

With reference to FIG. 23, the high anchor mooring system shown in FIG. 22 consists essentially of three mooring lines 62, pendent gravity devices 64, trip-plates 66, high anchor posts 82 and anchor piles 84 which arc located symmetrically on an outer perimeter of the mooring pattern. Each of the mooring lines 62 extends radially outward from a corresponding one of the fairleads 60 attached to the outer perimeter of the lower hull 4 to a corresponding one of the high anchor posts 82 coupled to a corresponding one of the anchor piles 84.

With reference to FIG. 24, the floating offshore wind turbine with the high anchor mooring system shown in FIG. 22 at a maximum horizontal offset is illustrated. For simplicity of this description, the high anchor mooring system is illustrated using two sets of high anchor posts 82A and 82B with corresponding anchor piles 84A and 84B fixed to the seabed 201, mooring lines 62A and 62B pendent gravity devices 64A and 64B, tri-plate 66A and 66B, and fairlead 60A and 60B in a two-dimensional arrangement. As the floating offshore wind turbine moves in a right direction to a maximum horizontal offset, the horizontal distance from the anchor pile 84B to the hull fairlead 60B increases, the mooring line tension force of the mooring line 62B increases as the V-shaped mooring configuration is getting shallower, and the tri-plate 66B moves up to an upper limit with the pendent gravity device 64B. In the meantime, the horizontal distance from the anchor pile 84A to the hull fairlcad 60A decreases, the mooring line tension force of the mooring line 62A decreases as the V-shaped mooring configuration is getting steeper, and the tri-plate 66A moves down to a lower limit with the pendent gravity device 64A just above the seabed 201. The mooring system restoring force is a resultant force of the horizontal component forces of the mooring line tension forces at the fairlead 60A and 60B of mooring lines 62A and 62B. The V-shaped mooring line configurations and mooring line angles of the mooring lines 62A and 62B can be obtained for a given horizon& offset based on geometry, and the mooring line tension forces and the mooring system restoring force can then be derived based on the submerged weights of the pendent gravity devices 64A and 64B, respectively.

With reference to FIG. 25, a schematic front view of another embodiment of the floating offshore wind turbine with the high anchor mooring system shown in FTG. 22 having a plurality of anchor piles 84 in a relatively deeper water depth is illustrated. With the deeper water depth, the mooring line 66 will have an increased mooring line angle and length of from the hull padeye 60 to the corresponding tri-plate 66 and an increased horizontal distance between the hull padeye 60 and the corresponding anchor pile 84 while the elevation of the high anchor post 82 from the seabed 201 remains the same as in FIG. 22. It is noted that this arrangement of the high anchor mooring system is not limited to 150m water depth and can be deployed to 200m water depth and beyond while maintaining a reasonably small mooring footprint on the seabed with the radius of the mooring circle less than the water depth. The above descriptions and figures are exemplary embodiments of the present invention and preferred main points of the mooring systems and methods for floating offshore wind turbines in a shallow water with a water depth range generally between 50 and 150 meters. However, the mooring systems are not limited to 150 meters water depth and can be deployed to 200 meters water depth and beyond. All modifications, equivalents, and alternatives to the above preferred embodiments are to be covered in the character and scope of the present invention.

Claims (20)

1. CLAIMSWhat is claimed is: 1. A mooring system for station-keeping of a floating offshore wind turbine, comprising: an anchor pile or a plurality of anchor piles fixed to a seabed, a floating body having an upper hull extending vertically above a waterline and a lower hull coupled to the upper hull below the waterline; a plurality of pendent gravity devices each having a submerged weight; a plurality of fairleads; a plurality of padeyes; a plurality of mooring lines, wherein each of the mooring lines having one end attached to a corresponding one of the pendent gravity devices from which running vertically upward through a corresponding one of the fairleads then extending to a second end, wherein each of the mooring lines having a constant tension force equal to the submerged weight of the corresponding one of the pendent gravity devices suspended in a mid-water above the seabed, wherein the pendent gravity devices move upward or downward when the floating body having a horizontal offset from an initial position.
2. 2. The system of Claim I, wherein having an anchor pile, a plurality of fairlead supports, and an anchor head structure coupled to a top of the anchor pile, wherein having each of the fairlead supports symmetrically coupled to an outer perimeter of the lower hull extending a distance radially outward.
3. 3. The system of Claim 2, wherein consisting essentially of at least three mooring lines, three padeyes, three fairleads, three pendent gravity devices and three fairlead supports forming a symmetrical mooring pattern, wherein each mooring line having one end attached to a corresponding one of the pendent gravity devices suspended below each of a corresponding one of the fairleads attached to each of a corresponding one of the fairlead supports from which running vertically upward through the corresponding fairlead then extending radially downward to a second end connected to a corresponding one of the padeyes symmetrically coupled to a top of the anchor head structure in a center of the mooring pattern which is an intersection of a vertical centerline of the floating body and the seabed.
4. 4. The system of Claim I, wherein each of the fairleads having a wheel with a grooved rim around which each of the corresponding mooring lines runs through from a first side to a second side acting to change a direction of the mooring line while maintaining a constant magnitude of the tension force of the mooring line, wherein the magnitude of the tension force of the mooring line on both sides of the fairlead equal to the submerged weight of the corresponding pendent gravity device.
5. The system of Claim 1, wherein having each of the pendent gravity devices consisting essentially of a large main gravity unit and a plurality of small secondary gravity units of preferably spherical shape suspended below the main gravity unit linked via a chain or rope; wherein having each of the main gravity unit a height of 1 to 3m with a cross section of square, circle, rectangle, or other shapes.
6. 6. The system of Claim 2, wherein having the anchor head structure consisting essentially of an upper section and a lower section of preferably of tubular pipes, wherein having the lower section with an adjustable length coupled to the top of the anchor pile at the seabed, wherein having the upper section comprising a tubular pipe with each of the padeyes coupled symmetrically to a top of the tubular pipe with a plurality of link plates connecting the padeyes inside the tubular pipe.
7. 7. The system of Claim 1, wherein having an anchor pile, a fairlead support structure and an anchor extension structure coupled to a top of the anchor pile at the seabed extending vertically upward to a substantial height below the lower hull, wherein consisting essentially of at least three mooring lines, three padeyes, three fairlcads and three pendent gravity devices forming a symmetrical mooring pattern, wherein each mooring line having one end attached to a corresponding one of the pendent gravity devices suspended in a mid-water below a corresponding one of the fairleads attached symmetrically to the fairlead support structure and running vertically upward through the fairlead then extending radially upward to a second end attached to a corresponding one of the padeyes coupled symmetrically to an outer perimeter of the lower hull, wherein having the fairlead support structure coupled to a top of the anchor extension structure located in a center of the mooring pattern which is an intersection of a vertical centerline of the floating body and the seabed.
8. 8. The system of Claim I, wherein having a plurality of anchor piles and a plurality of anchor head structures each coupled to a top of a corresponding one of the anchor piles, wherein consisting essentially of at least three mooring lines, three padeyes, three fairleads, three pendent gravity devices, three anchor piles and three anchor head structures forming a symmetrical mooring pattern, wherein each mooring line having one end attached to a corresponding one of the pendent gravity devices suspended in a mid-water below a corresponding one of the fairleads attached symmetrically to an outer perimeter of the lower hull and running vertically upward through the fairlead then extending radially downward to a second end connected to a corresponding one of the padeyes attached to a top of a corresponding one of the anchor head structures located symmetrically at a horizontal distance radially from a center of the mooring pattern which is an intersection of a vertical centerline of the floating body and the seabed.
9. 9. The system of Claim 8, wherein each of the anchor piles having a horizontal distance to the corresponding one of the fairleads less than or equal to a vertical distance from the waterline to the seabed.
10. 10. The system of Claim 1, wherein the mooring line can be of steel chain, steel wire or synthetic rope such as polyester or nylon or a combination thereof.
11. II The system of Claim 1 wherein the material of the pendent gravity device can be of steel, iron ore, concrete or other types of heavy materials or a combination thereof.
12. 12. A mooring system for station-keeping of a floating offshore wind turbine, comprising: an anchor pile or a plurality of anchor piles fixed to a seabed; a high anchor post or a plurality of high anchor posts; a floating body having an upper hull extending vertically above a waterline and a lower hull coupled to the upper hull below the waterline; a plurality of pendent gravity devices, each having a submerged weight; a plurality of anchor padeyes, each located at a height above the seabed; a plurality of tri-plates; a plurality of hull padeyes; a plurality of mooring lines, each having two segments linked by a corresponding one of the tri-plates connecting to a corresponding one of the hull padeyes at one end and a corresponding one of the anchor padeyes at a second end, wherein having each of the pendent gravity devices suspended in a mid-water below a corresponding one of the LH-plates linked by a chain or rope above the seabed, wherein the pendent gravity devices move upward or downward when the floating body having a horizontal offset from an initial position.
13. 13. The system of Claim 12, wherein having an anchor pile and a high anchor post coupled to a top of the anchor pile at the seabed, and a plurality of hull padeye supports, wherein having the high anchor post extending vertically upward from the seabed to a substantial height, wherein having each of the hull padeye supports symmetrically coupled to an outer perimeter of the lower hull extending a distance radially outward
14. 14. The system of Claim 13, wherein consisting essentially of at least three mooring lines, three hull padeyes, three anchor padeyes, three hull padeyc supports, three tri-plates and three pendent gravity devices forming a symmetrical mooring pattern, wherein each mooring line having one end attached to a corresponding one of the hull padeyes coupled to a corresponding one of the hull padeye supports running radially downward to a corresponding one of the triplates then extending upward to a second end connected to a corresponding one of the anchor padeyes attached symmetrically to a top of the high anchor post in a center of the mooring pattern which is an intersection of a vertical centerline of the floating body and the seabed, wherein each of the anchor padeyes having a higher elevation than the corresponding tri-plate, wherein each of the mooring lines having a V-shaped configuration.
15. 15. The system of Claim 12, wherein having a plurality of anchor piles and a plurality of high anchor posts, wherein consisting essentially of at least three mooring lines, three hull padeyes, three anchor padeyes, three tri-plates, three pendent gravity devices, three anchor piles and three high anchor posts forming a symmetrical mooring pattern, wherein each mooring line having one end attached to a corresponding one of the hull padeyes and running radially downward to a corresponding one of the tri-plates then extending upward to a second end connected to a corresponding one of the anchor padeyes attached to a top of a corresponding one of the high anchor posts coupled to a top of a corresponding one of the anchor piles located at a horizontal distance radially outward from a center of the mooring pattern which is an intersection of a vertical centerline of the floating body and the seabed.
16. 16. The system of Claim 15, wherein each of the anchor piles having a horizontal distance to the corresponding one of the hull padeyes less than or equal to a vertical distance from the waterline to the seabed.
17. 17. The system of Claim 15, wherein each of the high anchor posts extending vertically upward from the seabed to a substantial height, wherein each of the corresponding anchor padeye attached to the top of the high anchor post having a higher elevation than the corresponding triplate, wherein each of the mooring lines having a V-shaped configuration.
18. 18. A method for station-keeping of a floating body in a water, comprising the steps of: (a) providing a mooring system comprising an anchor pile fixed to a seabed, an anchor head structure, a plurality of mooring lines, a plurality of fairleads, a plurality of fairlead supports; a plurality of padeyes and a plurality of pendent gravity devices below a waterline: (b) attaching the anchor head structure to a top of the anchor pile: (c) attaching each of the padeyes symmetrically to a top of the anchor head structure; (d) attaching each of the fairlcad supports symmetrically to an outer perimeter of a lower hull of the floating body, wherein extending a distance radially outward: (e) attaching each of the fairleads to each of the corresponding fairlead supports; (f) setting the anchor pile at an intersection of a vertical centerline of the floating body and the seabed; (g) connecting a first end of each of the mooring lines to each of the corresponding pendent gravity devices; (h) running each of the mooring lines vertically upward through each of the corresponding fairleads; ( ) connecting a second end of each of the mooring lines to each of the corresponding padeyes: (j) generating a tension force for each of the mooring lines by suspending each of the corresponding pendent gravity devices in a mid-water below each of the corresponding fairleads, wherein the tension force of the mooring line on both sides of the fairlead equal to the submerged weight of the pendent gravity device, wherein the fairlead having a wheel with a grooved rim through which the mooring line runs, and wherein the wheel acting to change a direction of the tension force applied to the mooring line while maintaining a magnitude of the tension force according to a pulley principle.
19. 19. The method of Claim 18, wherein for simplicity having a mooring system consisting of two mooring lines with two corresponding pendent gravity devices in a two-dimensional arrangement opposite to each other, comprising the steps of (a) suspending each of the pendent gravity devices in the water at an initial depth approximately halfway between the seabed and a bottom of the floating body; (b) adjusting a height of the anchor head structure and a length of each of the fairlead supports; wherein forming a mooring angle between each of the mooring lines and the waterline in a range of 30 to 60 degrees; (c) having a horizontal offset of the floating body, wherein causing the mooring line length between the corresponding fairlead and the anchor pile to increase and the corresponding pendent gravity device to move up for a first mooring line, and the mooring line length of between the corresponding fairlead and the anchor pile to decrease and the corresponding pendent gravity device to move down for a second mooring line; wherein the mooring line angle of the first mooring line becoming smaller and the mooring line angle of the second mooring line becoming greater; wherein causing a greater horizontal component force for Lhe first mooring line and a smaller horizontal component force for the second mooring line than the respective initial horizontal forces of the first and second mooring lines without the horizontal offset; (d) causing a horizontal resultant force of the horizontal component forces of the mooring line tension forces of the first and second mooring lines; (e) having the horizontal resultant force in an opposite direction of the horizontal offset, wherein proving a horizontal restoring force for station-keeping of the floating body.
20. 20. The method of Claim 18, wherein for the floating body in a water with a water depth less than 60 meters, comprising the steps of (a) providing a plurality of large gravity units; (b) providing a plurality of small gravity units of preferably spherical shape; (c) forming a plurality of pendent gravity devices, each consisting essentially of a large gravity unit and a plurality of the small gravity units attached to a bottom of the large gravity unit by a plurality of short chains; (d) providing a greater safety factor for each of the mooring lines attached to a corresponding one of the pendent gravity devices; (c) suspending each of the pendent gravity devices in the water at an initial distance above the seabed equal to approximately 10% -15% of the water depth; (1) adjusting a height of the anchor head structure and a length of each of the fairlead supports; wherein forming a mooring angle between each of the mooring lines to the waterline in a range of 30 to 60 degrees;