NO347179B1

NO347179B1 - A mooring system for a plurality of floating units

Info

Publication number: NO347179B1
Application number: NO20200826A
Authority: NO
Inventors: Niklas Norman
Original assignee: Semar As
Priority date: 2020-07-14
Filing date: 2020-07-14
Publication date: 2023-06-19
Also published as: WO2022013145A1; EP4247701A1; WO2022012769A1; NO20200826A1; WO2022106619A1; KR20230107268A

Description

Title

A mooring system for a plurality of floating units

Field of the invention

The present disclosure relates to a mooring system for an array of floating units, for example a farm of floating wind turbines.

Background of the invention

A mooring system of a floating unit has a main purpose: station-keeping, i.e. make sure the offshore floating structure stays within the area required for intended operation. To achieve its main purpose, the mooring-system needs three elements.

- a connection point to the seabed, also called an anchor,

- line elements, transferring the environmental forces applied on the floating unit to the anchor at the seabed. Mooring lines need to transfer the average of the environmental dynamic forces acting on a floating offshore structure during the worst design storm down to the anchor, with as low dynamic force-amplification as possible

- An element providing flexibility to the mooring system: mooring lines are generally not designed to stop the wave motions of big volume offshore structures, simply because this would impose excessively high dynamic peak loads to the mooring lines and to the offshore structure during the worst design storm. Successful mooring of offshore big volume structures therefore generally depend heavily upon sufficiently large flexibility in the mooring system, so that the dynamic peak loads due to the cyclic wave-motions, excessive current or excessive wind to which the floating structure is submitted during the worst storm, are handled by an element providing flexibility in the mooring system, saving the mooring lines for high dynamic stretching peak loads which may trigger rupture or “snapping “of the line.

Catenary and taut mooring are the usual mooring concepts used for mooring of offshore structures.

Taut mooring consists of straight synthetic lines (most often polyester) or metal wires where the element providing flexibility to the mooring system is the elasticity of lines: the lines stretch when the environmental forces drive the floating unit away from the anchor point. When stretched, the elastic reaction forces participate to restore the floating unit to its initial position. Taut systems are normally used in deep water, where the length of the line enable extended stretching without overloading the lines.

Catenary systems consists of heavy steel-chains (possibly reinforced by clump-weights on chains or other type of lines) where the element providing flexibility to the mooring-system is based on the gravity-induced mostly vertical geometric adaptation of the chains and clump-weights. When the environmental forces drive the floating unit away from the anchor, the chains are put under tension, and a larger part of the chain is mobilized and stops resting on the seabed resulting in an increase of the apparent weight of the catenaries, participating to restore the floating unit to its initial position.

Catenary lines are by far the most widely used of the two above described mooring concepts. Sometimes clump-weights are introduced for increased “catenary effect”, and sometimes both synthetic taut-line-segments and catenary-line-segments, are combined in the same line. As synthetic lines are not allowed to get in contact with the seabed, small buoys may be attached to a line in order to lift the synthetic line enough for it not to touch the seabed.

Both catenary and taut lines mooring concepts are based on elements for providing stiffness which are individual, i.e. at the level of each moored floating unit.

Both catenary and taut lines mooring concepts present restoring forces where the horizontal component of the restoring force is limited compared to the vertical component of it.

According to its abstract, CN 111071400 A relates to a floating type offshore wind plant and a mooring integration method of the floating type offshore wind plant. The floating type offshore wind plant comprises: a first anchorbase; at least two floating fans, wherein the floating fans float on the sea surface; and at least two floating fans which are arranged around the same first anchor foundation at intervals, each floating fan is connected with a plurality of mooring lines, and one mooring line on the at least two floating fans is connected with the same first anchor base. The mooring line of the at least two floating fans is integrated on the same first anchor base, so that the use amount of the first anchor base is effectively reduced, the manufacturing and mounting cost of the first anchor base is effectively saved, and meanwhile, the sea area range occupied by the floating offshore wind plant is greatly reduced. A shared anchor point method is adopted in a shallow water area, so the anchor consumption is effectively reduced, and the anchor cost is effectively saved; and an anchor chain sharing method is adopted in a deep water area, so the use amount of anchor chains is effectively reduced, and the cost of the anchor chains is effectively saved.

According to its abstract, the paper NISSA, M. CHOW, .C., “Mooring system design for a floating wind farm in very deep water”, European Wind Energy Master Thesis, Norwegian University of Science and Technology, January 2019, https://ntnuopen.ntnu.no/ntnuxmlui/ handle/11250/2580746, presents a thesis project aiming to develop a working mooring system at depth of 600 m in the Norwegian North Sea, and then investigates the possibility of shared anchors in a wind park with this mooring system. The DTU 10MW reference wind turbine atop a classic spar substructure is used.

According to its abstract, the paper RINGSBERG, J. ET. AL. “Comparison

of Mooring Solutions and Array Systems for Point Absorbing Wave Energy Devices”, paper no. OMAE2018-77062, OMAE, 062018,

https:// https://asmedigitalcollection.asme.org/OMAE/proceedingsabstract/OMAE2018/51326/V11AT12A037/274129, presents a study focusing on design of mooring solutions and compares array systems of a specific floating point-absorbing wave energy converter (WEC) developed by the company Waves4Power.

According to its abstract, EP 0407662 A1 relates to a device for positioning of a buoy body, comprising a ballast weight in use positioned under said buoy body and being provided with anchoring means, and coupling means connecting said ballast weight to said buoy body. The mean horizontal cross sectional dimension of the buoy is smaller than the height of the buoy.

According to its abstract, EP 2604501 A1 relates to a system of floating and weight-stabilized wind turbine towers with separately floodable compartments and aerodynamic overwater encasement and the appertaining semisubmersible mooring structures including anchorage on the seabed, a horizontally floating underwater mooring meshwork and an actinomorphic buoycable-mooring to the wind turbine towers. Furthermore, EP 2604501 A1 relates to a method to transport wind turbine towers from the manufacturing plants on-shore to the final off-shore destination (energy production sites) by towing the vented towers in a horizontal position with towboats. Finally, EP 2604501 A1 includes the method to erect the wind turbine towers at the final destination, including the assembly of the wind turbines.

According to its abstract, CN 110654510 A relates to an offshore wind power platform group sharing mooring. The offshore wind power platform group is of a honeycomb array structure, a honeycomb array comprises at least one hexagonal unit, each hexagonal unit comprises three floating offshore wind power platforms and three mooring berths, the floating offshore wind power platforms and the mooring berths are alternately arranged at the corner point positions of the hexagonal units, and the adjacent floating offshore wind power platforms and mooring berths are connected through mooring cables. According to the offshore windpower platform group sharing the mooring berths, a mooring berth sharing mode is adopted, pile foundations for mooring are greatly reduced, and the installation cost of a whole offshore wind power field is reduced.

The main challenges to be addressed designing a mooring system are the following. They are discussed in general for floating units, and in particular for floating wind turbines (FWT).

- Flexibility of the mooring system/mooring line tension vs floating unit offset: the mooring system of an array of floating units needs to be flexible even after the full static environmental loads are connected to the mooring system. This is challenging, in particular in shallow waters where the lines are shorter and less capable of absorb displacements and tensions, and in the case of wind farms because the environmental static wind-force is large for floating wind turbines (FWT) as they are designed to “catch” the wind. The rigidity of a mooring line under tension submitted to the additional environmental dynamic forced wave-motions increases rapidly, often exponentially. To prevent high peak-loads, fatigue and risks of rupture due to these wave motions, the rigidity of the line should not increase too much when the floating unit offset increases slightly (and repeatedly) due to forced wave-motions of the floating (in a graph X=floating unit offset/Y=rigidity of the line, the slope of the rigidity curve, positive, should be low). Normally for gravity-based (= catenary) mooring systems, the way to achieve this is to add even more weight to the mooring line (here, a chain) by selecting thicker chain or heavier clump weight so that the mooring line keeps a marked curvature and is not submitted to excessive tension. The problem, however, is that by adding more weight, the mooring line tension at the floating unit end increases, simply due to the extra weight of the chain. As a given forced wave-motion is easier to adapt to the longer the mooring line is, taut lines which repeatedly stretch and relax are preferred for deeper waters.

- Sharing of mooring components, optimal design: sharing of mooring components has the potential for drastic cost cuts. Traditional mooring-systems make anchor sharing challenging for most water depths as the general rule resulting from 1- required mooring flexibility and 2- required FWT spacing for hydrodynamic efficiency are generally not compatible. Arbitrage results in excess mooring costs or reduced power production efficiency. As a result, it has been common to design three anchor points per FWT. For the traditional catenary and taut lines concepts discussed above, there is no sharing of means providing flexibility, they are individual.

- Chain production costs and capacity : catenary (chains) lines are the most commonly used today. Heavy chains of high-grade steel are expensive and heavy. Production capacity of chains is limited, simply because the casting process for such heavy chains is slow.

- Synthetic line stiffness-elongation challenges: over time, in particular when submitting them to many stretching cycles, taut fibre or metal wires show elongation and stiffening, increasing risks of rupture.

- Standardisation of components and scalability: one-off mooring designs are expensive.

Standardisation of mooring components is crucial for reducing capex and in less extent opex of floating windfarms. Components which production is scalable should be preferred. Production of heavy chains is little scalable.

- Damage to the marine environment: heavy mooring chains damage the seafloor when lifted up and down or sweeping the seafloor.

- Installation costs and safety: high requirements to pre-tension of the system and the sheer weight of chains require costly installation equipment and procedures. The weights and forces involved present significant risk.

- Maintenance costs and safety: high pre-tension and heavy chains require costly and risky maintenance when replacement of components or dethatching of FWT for service.

In the sub-section “Floating Wind Turbine Technology - A Power Source For 2050” of it its report “Global electrification outlook 2050: DNV GL: Electrifying The Future”, Høvik, 2014, DNV-GL represents in page 38 a concept to address some of the above listed challenges. For illustration, this document is supported by a video published by DNVGL at Youtube: https://www.youtube.com/watch?v=sy1z0Tz1Knw.

In its document, DNV-GL describes sharing anchor points at the seabed between six FWTs, thus reducing the ratio anchor per turbine. It also describes and illustrates in the above-mentioned Youtube video, a solution for deeper waters where the FWTs are connected to “buoyancy elements that are connected to the seabed”.

This concept can be explained as following. The optimal spacing and positioning between FWTs is primarily governed by aerodynamic considerations vs area usage optimization. When using the traditional catenary or taut lines mooring concepts together with an optimized FWT spacing and positioning, the mooring lines would be crossing each other when water gets deeper, thus prohibiting an optimal aerodynamically efficient pattern. Another challenge when water gets deeper using catenary mooring lines, is the weight of these chain lines. To address these two challenges, DNV-GL describes submerged tension leg buoys at appropriate depth for providing shared anchor-point of catenary mooring lines to neighbouring FWTs, thus reconstituting an artificially shallow seabed with lifted anchor points.

The video illustrates well the concept by illustrating the migration towards deeper waters, and the substitution of the seabed anchor points with lifted anchor points.

The DNV-GL concept presented for deep waters addresses a certain number of the above listed challenges, such as sharing of mooring components (lifted anchor points), good aerodynamical design (buoys allow keeping for deep waters the design of shallow waters), saving on chains (by artificially lifting the anchor points, length of chains is reduced), standardisation of components and scalability to some extent only (the length of catenary lines between the FWTs and the lifted anchor points may remain similar between shallow and deep waters), reduced damage to the marine environment (as the “lifted” catenary mooring lines do not contact the seabed), reduced installation and maintenance costs for less equipment in more shallow water , increased safety (shorter and probably lighter chains).

However, as clearly illustrated in the print and the video (see curvatures and profiles of the mooring lines), the system presented by DNV-GL :

- still makes intensive use of catenaries in order to obtain the flexibility required to resist the environmental dynamic forces of wave motion on the mooring lines.

- Presents thus a flexibility solution which is individual, i.e. designed at the level of each floating unit.

There is a need for a lighter and cheaper mooring system which enables a limited if not absent use of catenaries, expensive to produce and install, and which may damage the seabed.

There is a need for a cheaper mooring system which enables the use of cheaper lighter fibre or metal wires, comprising integrated means providing mooring flexibility, such that cheaper lines with less demanding specifications can be used without sacrificing safety and robustness.

The is a need for a more efficient mooring system where the horizontal component of the restoring force is large with regard to the vertical one.

There is a need for a cheaper mooring system where the means providing flexibility are shared between floating units.

There is generally a need for a mooring system which is cheaper and faster to install and maintain, in order to make offshore wind more competitive and reduce global production of CO2.

Brief summary of the invention

In view of the above, an object of the present invention is to provide a flexible and efficient mooring system for a plurality of offshore floating units which enables the use of lighter and cheaper material and components, reduces the footprint on seabed, and is safer and cheaper to install.

This objective is achieved by a mooring system according to claim 1.

As such the present invention relates to a mooring system for an array of floating units, comprising

- a plurality of floating units,

- a plurality of buoys,

- a plurality of connection points to the seafloor such that for each buoy, there is at least a connection point to the seafloor,

- one or more tensioned lines linking each buoy to its one or several connection points, - A plurality of lines between the floating units and the buoys such that each floating unit is connected to several buoys and each buoy is connected to several floating units,

According to the present invention,

- the lines are slack lines made of fibre, metal wire or a fibre-metal composite, and - the buoys account for the major part of the mooring system flexibility.

The buoy will be able to move like a pendulum around the connection point. When a floating unit is submitted to environmental dynamic forces driving it away from its static position, the buoy will be displaced in the same direction, thus moving deeper. As a result, the buoyancy forces will increase, and so will participate to restoring forces on the floating unit.

The shared buoys of the system account for more than 50%, 75% or 85%, or even more than 90% or 95% of the incremental mooring system flexibility for any floating unit.

The system may be designed so that the buoys in direct contact with a floating unit provide nearly all the buoyancy flexibility, i.e. the influence of the buoys not in direct contact with the FWT account for a negligible effect. In this case, the buoys in direct connection with the FWT account for more than 50%, 75% or 85%, or even more than 90% or 95% of the incremental mooring system flexibility for any floating unit.

The system of the invention is based on light lines, fibre lines, metal wires or composite fibremetal lines. These are lighter, cheaper, easier to install and replace.

Optionally, the buoy can be positioned at or close to the surface, between a position piercing the water surface and a position deep enough to provide safe vessel sailing.

In such case, the slack lines are close to the horizontal, and the restoring force from the system is close to the horizontal. Such system is then very efficient in terms of station-keeping. The horizontal component of the restoring force represents at least 50, 75 or 85%, or even at least 90 % or 95 % of the total restoring force.

Optionally, the buoy is a spar buoy. This provides a more robust system, with improved restoration.

The mooring system of the invention may be well adapted to be stationed in a configuration based on the repetition of a pattern.

Optionally, a buoy is connected to 2 to 6, preferably 3 to 6, even more preferably 3 floating units.

Optionally, a floating unit is connected to 3 to 6 buoys, preferably 3 buoys.

Optionally, the pattern is hexagonal, and the six angles of the hexagons are filled alternatively with a floating unit and a buoy.

Optionally, the hexagonal grid may be formed by equiangular hexagons, like a honeycomb.

Optionally, the array of floating units is a farm of floating wind turbines.

Figures

Fig.1: a schematic representation of a prior art mooring system of a FWT by taut lines, a- under static forces with low wind, b- when submitted to environmental dynamic forces

Fig.2: a schematic representation of a prior art mooring system of a FWT by catenaries, a- under static forces with low wind, b- when submitted to environmental dynamic forces

Fig.3: a schematic representation of a prior art mooring system of a FWT by catenaries with lifted anchor points (DNV-GL reference), a- under static forces with low wind, b- when submitted to environmental dynamic forces

Fig.4: a schematic representation of an embodiment of the flexible mooring system of the invention, with two FWT sharing one anchor point a- under static forces with normal design wind, b- when submitted to environmental dynamic forces

Fig.5: a schematic representation of an embodiment of a FWT farm using the flexible mooring system of the invention in a honeycomb pattern

Detailed description of the invention

The invention will be described for the case where the floating units are floating wind turbines. However, the invention applies to all sorts of floating units and their arrays.

Fig.1 illustrates how a taut lines mooring system for a FWT 25 of the prior art reacts to environmental dynamic forces such as in a storm, with wind W, rough waves F and currents C. The mooring system comprises an anchor 21 at the seabed, a taut line line 26 mooring the floating unit 25 to the seabed. When in b) these forces drive the FWT 25 away from its initial position with regard to the anchor 21 in a), the taut line 26 is stretched thus allowing displacement of the FWT 25. The elastic reaction forces in the line 26 tend to restore the FWT 25 back to its position at a). The restoring forces of taut lines have a dominant vertical component, and a limited horizontal one. Taut lines are repeatedly stretched by dynamic forces, which over time cause elongation and stiffer properties.

Fig.2 illustrates how a catenary mooring system for a FWT 35 of the prior art reacts to environmental dynamic forces such as in a storm, with wind W, rough waves F and currents C. The mooring system comprises an anchor 31 at the seabed, a catenary line 36 mooring the floating unit 35 to the seabed. When in b) environmental dynamic forces drive the FWT 35 away from its initial position with regard to the anchor 31 in a), the catenary 36 is pulled thus allowing displacement of the FWT 35. At the same time, the weight of the catenary 36 – increased by the portion lifted from the seabed when straightened, tend to restore the FWT 35 back to its position at a). The restoring forces of taut lines have a dominant vertical component, and a limited horizontal one: catenaries need thus be heavy. They are expensive, slow to produce, and they also damage the seabed by sweeping the seabed.

Fig.3 illustrates how a catenary mooring system for a FWT 45 of the prior art, as described in the above referenced DNV-GL documents, reacts to environmental dynamic forces such as in a storm, with wind W, rough waves F and currents C . The mooring system comprises an anchor 41 at the seabed, a buoy 43, an anchoring line 42, a catenary line 44 mooring the floating unit 45 to the buoy. When in b) environmental dynamic forces drive the FWT 45 away from its initial position with regard to the lifted anchor 41 in a), the chain 36 is pulled and slightly straightened thus allowing displacement of the FWT 45. However, the weight of the catenary and the tension on it tend to restore the FWT 45 back to its position at a). The advantage here compared to figure 2 is that the catenaries do not sweep the seabed, and that their linear weight and length are less.

Fig.4 illustrates an embodiment of the invention. The mooring system 10 comprises an anchor 1 at the seabed, a buoy 3, a tensioned line 2 mooring the buoy 3 to the anchor 1. Here, two floating units, the windmills 5, are moored to the buoy 3 by slack lines 4, of a non-catenary type such as a fibre line or a metal wire or other types, but not chains. When environmental dynamic forces such as in a storm, with wind W, rough waves F and currents C drive the downwind FWT 5 away from its initial position with regard to the anchor 1 in a), the buoy is displaced in the same direction. As the buoy 3 is connected to the anchor point 1 and moves like a pendulum, deviating the buoy from the vertical increases its depth, and thus its restoring buoyancy. As a result, the restoring forces increase, and tend to bring the FWT 5 back to its initial position at a). The horizontal component of the restoring force in the present invention is dominant, the vertical one is limited. The horizontal component may thus represent at least 50, 75 or 85%, or even at least 90 % or 95 % of the total restoring force. That is, the present invention is very efficient in terms of geometrical horizontal restoring forces.

Additional tension and stretching on the slack lines 4 are very limited if not negligible, and compared to prior art, the mooring system gains in flexibility, weight, costs, safety, scalability, robustness.

In this case, and contrary to the cases of fig.1 to 3 where the restoration means were individual to each floating unit, the means bringing restoration is shared between the floating units, it is common to them. The flexibility and restoration capacity of mooring lies in the system design, more specifically at the level of the buoy. We have sharing of the buoy, and a very scalable system. Since elasticity requirements on the slack lines 4 are limited, cheaper lines can be used, and be standardized. With the present invention, one may imagine that from wind farm to wind farm, and for a given FWT type and meteorological/aerodynamical conditions, it is the buoy design which will vary most and accommodate the different variations.

Based on Fig.4, we will discuss embodiments of the present invention.

Whereas the system of the invention is well fitted for floating wind turbine (FWT), it may also apply to any floating structure which is part of an offshore array of units needing mooring, such as offshore oil and gas platforms or vessels, seabed mineral exploitation units, solar panel floating units etc…

In this detailed description however, we will mostly illustrate the case of floating wind turbines (FWTs).

The connection point or anchor 1 needs to handle the constant uplift of buoy 3, and its design will also depend on seafloor soil-conditions. It can strictly be an anchoring means, or it can be any means for fastening or connecting to the seabed, for example a block of concrete etc… The solution to adopt will be known to the skilled person. Compared to a catenary mooring design of the prior art for similar FWT and local environmental conditions, Anchor 1 will generally be designed for higher static forces and lower dynamic forces.

The connection point may also be multiple smaller connections points, made solidary locally or at the level of the one or several lines to the buoy 3.

The buoy 3 is preferably positioned at the water surface, piercing it, or below but preferably close to the surface. One reason is that the closer it is to the surface, the longest the tension line 2 to the anchor 1 will be, and the better system flexibility the buoy will provide.

The buoy 3 is connected to the anchor 1 by a tension line 2, also called vertical line in the description, even if it is not necessarily strictly vertical. Line 2 will also be described as the anchoring line or anchor line. The buoy 3 is designed with excess buoyancy, and line 2 shall have no slack. Line 2 is preferably connected by “moment-free” connections, such as a hinge, to the anchor, and to the buoy preferably at a point below the buoy 3.

The anchor line 2 will preferably be non-catenary, made of one or several fibre lines or metal wires.

The anchor line could also be a chain, in which case the buoy 3 will need to be designed taking into account the extra chain weight.

The buoy 3 will be able to move as a pendulum around the connection point. Its position will be comprised in the virtual sphere centered on the anchor point 1 and with a radius corresponding to the length of the tension line 2.

In Fig.4, the buoy is linked to two floating units. It may in other embodiments be connected to multiple floating units, typically 3 to 6. Or it may be connected in some cases to only one.

The buoy 3 is preferably partly submerged piercing the water surface, but it may also be submerged. Partly submerged buoys will reduce the level of constant uplift force at the anchor in gentle weather and allow for shallower windfarms. FWT farms extend over huge surfaces, and having the slack lines under sailing depth of sailing vessels may be required, in particular for operation and maintenance vessels. Thus, placing the buoy 3 at 20, 30, 40 or 50 meters depth may provide a safe sailing environment.

The buoy 3 should always generate sufficiently high buoyancy force to avoid any slack in the anchor line 2, which elongation shall be monitored over time.

In one embodiment, the buoy 3 is of a SPAR type. It is designed to minimize wave forces and added mass forces acting on it. When piercing the water surface, the SPAR buoy design will help adjust buoyancy restoration by adding the extra buoyancy of the statically emerged part of the buoy which goes submerged under strong dynamic forces. The buoy is designed to rotate (pitching) at small angles to minimize fatigue challenges. It is also designed to provide optimal line-stiffness-properties to the FWTs to be moored without the need for additional gravityelements such as clump-weights, chain or any special requirement on the elasticity properties of the slack lines 4.

Structurally, the buoy 3 is designed so that the inside air pressure is higher or equal to the outer water-pressure. Internal over-pressure together with the moment-free coupling below the buoy allow this structure to be light and low cost. In a preferred embodiment, the buoy is a steel structure. However, the mooring system 10 of the invention could work with a buoy 3 of the same shape but another material such as an elastomer, or with a buoy with a different shape. The dimensions of the buoy will depend on turbine size and other parameters like water depth and meteorological/ocean conditions. For turbine sizes known or planned today, in the range of 15 MW, and for SPAR type buoys, the diameter would typically be in the range of 3 and 8 meters and the height in the range of 20 to 60 meters.

The buoy 3 connects to one or several FWTs via lines 4. These are also called horizontal lines, even though they may not be strictly horizontal, or slack lines. They connect the FWTs 5 to the buoys 3, at or close to surface. The lines are preferably light, made of fibre lines, metal wire, or a composite of fibre and metal.

As explained above for the buoys, the slack lines 4 may need to be kept at a certain depth of for example 20, 30, 40 or 50 m, in order to keep the area safe for sailing.

The slack lines 4 connecting FWTs 5 to the buoy 3 are not necessarily strictly horizontal. For the first, since the lines are flexible, they may sometimes be meandering in the water. In addition, the system may easily accept an angle β with the horizontal of for example less than 35 or 25 degrees (angle defined by a segment joining attachment of line to FWT and to buoy), or less than 20, 15, or preferably less than 10 or 5 degrees.

To keep its efficiency, the system is designed for the anchor line 2 and the FWT slack line 4 to keep an angle α far from 180 % (in line). At maximum environmental forces, the buoy should be designed not to allow an angle α superior to 145 or 135. even preferably 125 or 115 or even more preferably 105 degrees.

Because the slack lines are horizontal or close to horizontality, the main component of the restoring forces on the FWTs is horizontal. As restoring forces result mostly from buoyancy forces at the buoy, tension on the fibre or metal wire lines remain limited. As a result, the line material and line properties – in particular elasticity and weight - are of relatively little importance as long as the lines are sufficiently strong. Such limited requirements allow for significantly cheaper lines, in particular compared to chains.

The mooring system of the invention would very function with rigid lines between the FWTs and the buoys. In practice, slack lines 4 will show some elasticity. However, the buoys 3 can be designed to take over more than 50%, 75% or 85%, or even 90% or 95% of the mooring system flexibility. Even though quantifying flexibility and its share between the slack line and the buoy is not easy to measure on site, it is possible to have a reasonably good estimate by calculations and modelling. One alternative is to model the incremental displacement of the FWT at the extreme design case, i.e. the worst storm the mooring system is designed for, also called Ultimate Limit State, where the offset of the FWT is at its maximum. It can be estimated which % of the incremental displacement is accounted for by the buoy system, and which % is accounted for by the stretched slack line.

In a preferred embodiment, the lines should preferably not be buoyant to ease submersion and provide sailing depth for passing ships.

Horizontal line-pre-tension can be adjusted, if required, to increase FWT yaw stiffness or other properties, preferably by adding weight to the horizontal line segments 4. As lines connect to the FWT relatively horizontally, all line pre-tensioning adds on to the horizontal pre-tension, as opposed to concepts with steep gravity mooring lines (for example catenaries) with only a small component of line tension being horizontal and thus contributing to yaw-stiffness.

The horizontal lines 4 must be long enough to ensure that all environmental forces from any FWT is transferred to the closest one or two anchored buoys. This way, accumulating forces throughout the horizontal grid are avoided. Furthermore, any interaction grid-dynamical effects are avoided or minimized.

The mooring line system of the invention can be applied for water depth from around 40 m to about 2000 m, more often 80 to 1000 meters. Flexibility with regard to water depth distinguishes the invented mooring concept from other mooring systems of the prior art. In shallow waters, the mooring system of the invention allows greater flexibility compared to traditional mooring systems that tend to get too stiff when rough weather hits. It must be however assured that the vertical lines do not elongate throughout the lifetime to an extent where the vertical line can get slack.

The mooring system of the invention has no limitations in terms of horizontal line-lengths in order to function well. Their lengths can be decided according to the ideal aerodynamic FWT-spacing. Typical lengths of horizontal line segment can be in the range from 400 to 2000 meters.

One or several FWTs can be connected to a buoy, for example between 1 and 6, preferably between 2 and 5. In a preferred embodiment, 3 FWTs connect to a shared buoy. This preferred configuration optimizes horizontal force distribution on the anchor, and limits possible resonance between oscillations of the connected FWTs when too many FWTs are connected to a buoy. Sharing anchor has also a clear economical advantage.

A given FWT connects to several buoys, preferably between 3 and 6. In a preferred embodiment, a FWT connects to three anchors 1.

The mooring system find its best use for an array of floating units, such as a farm of FWTs. An important aspect is that thanks to the nature and flexibility of the design, and in particularly the absence of lines crossing underwater, the array of floating units, for example the wind farm, can be optimized for aerodynamics, capturing as much wind energy as feasible, whatever the depth.

Another important aspect is that the flexibility means are shared, that they allow the generalized use of slack lines, thus making wind farms less expensive.

% flexibility can be expressed for the whole system, i.e. the buoys in the system account for the the given % flexibility for any FWT.

In an embodiment, the mooring system is designed such that flexibility for a given FWT is accounted for essentially (say more than 75, 85 or 90 %) by the buoys in direct connection with the FWT. In other words, the buoys further away, which are not in direct contact with the FWT play a negligeable (less than 25, 15 or 10 %) role in the flexibility for the given FWT.

In a preferred embodiment, the array of FWTs and buoys are stationed according to a configuration with a repeated pattern. For example, seen from above, they may represent the angles of a grid of polygons. If the slack lines are visible from above, they will then materialize the edges of the polygons.

A hexagonal pattern is such an embodiment, as it has generally an optimal aerodynamic capture configuration. It may be equiangular such as the honeycomb, but does not need to be. It may also be isogonal, thus allowing the hexagon to be flattened and reduce its number of symmetries.

Fig.6 shows such a honeycomb configuration, where the horizontal lines segments represent the edges, and the FWTs or the buoys, alternating, represent the angles. A unit hexagon – as marked by the slack lines – has 3 angles with a FWT in alternance with 3 angles with a buoy. One turbine is connected to three buoys, and one buoy is connected to three turbines. This regular equiangular honeycomb pattern with 50/50 buoys and FWTs, where the horizontal lines have similar lengths also give favorable accidental load cases properties compared to other patterns:

● No collision between buoys or FWTs with one line-break

● If a horizontal line breaks, no increased maximum line tension in the remaining lines.

At the periphery or borders of the Fig.6 and all other pattern-based configurations, the symmetry and structure may be somehow degraded.

For example, there may not be anymore a complete hexagon. Whereas a FWT will generally need several buoys to connect to, buoys connecting a fixed number of FWTs in the core of the grid, may connect to less FWTs, or even only one.

Since the underwater flexible mooring system of the invention does not bring limitations on the aerodynamic design above the water , the method of designing an offshore wind farm with the flexible mooring system of the invention is uniquely simple in terms of sequences:

● From the aerodynamics, define the ideal production configuration: characteristics of turbines, their number and positioning

● Design the characteristics of the tension-leg buoys, size, anchoring line

● Design the anchor characteristics, such as type and anchoring capacity

● Define the required characteristics of the horizontal slack lines.

With the mooring system design of the present invention, there is no general geometrical design limitation of the type ratio depth/distance floating unit to connecting point (or length of the catenary). For the present invention, this ratio may cover a wide range of values, for example less than 1/3, 1/4, 1/5 or less than 1/6 for shallow water, or more than 1/3, 1⁄2 or more than 1 for deeper water.

The flexible mooring system of the invention uses lighter lines, less mileage of lines thanks to anchor and buoy sharing, and scaled up components. This system is easier, cheaper and safer to install than prior art concepts. The vessels used are lighter, installation is faster.

None of the individual components of the flexible mooring system is totally new, and a skilled person would probably be able to define a good installation sequence. We are here suggesting one.

1. Installation of all anchors.

o Install anchors

o ballast installed on top of anchors if gravity anchor is selected.

2. Installation of vertical lines

o Bottom part of line connected to anchor, with pick-up buoy in upper end.

3. Installation of all buoys

o Install all tensioned buoys using for instance a Jumbo-type vessel. Part of each horizontal line segment pre-installed at connection point below each buoy, and attached above water-line for later ease of FWT-hook-up.

o Part of buoy flooded during up-ending and hook-up of vertical line. o De-ballasting water from the TLSB to gain sufficiently level of vertical pretension in vertical lines. De-ballasting can be done simply by pump in air that will push the water out.

4. Hook-up of FWT.

o Remaining part of horizontal line segments pre-connected at FWTs and attached above water line.

o FWT towed to position

o Hookup of all horizontal lines on installation vessel deck, above the water line, between buoys and FWTs. Line tension during this hook-up will be low in gentle weather and be a low risk operation compared to gravity-lines with high tension.

5. Inter array cable installation

o Direct hanging cables below sailing-depth. With some floating elements if shallow water.

o Cable guided along the horizontal slack lines.

Claims

Claims

Claim 1

A mooring system (10) for an array of floating units, comprising

- a plurality of floating units (5),

- a plurality of buoys (3),

- a plurality of connection points to the seafloor such that for each buoy (3), there is at least a connection point to the seafloor,

- one or more tensioned lines (2) linking each buoy to its one or several connection points,

- a plurality of lines (4) between the floating units (5) and the buoys (3) such that each floating unit (5) is connected to several buoys (3) and each buoy (3) is connected to several floating units (5),

- the lines (4) are slack lines made of fibre, metal wire or a fibre-metal composite, characterized in that

- each buoy (3) is able to move like a pendulum around its connection point to the seafloor such that when a floating unit (5) is submitted to environmental dynamic forces driving it away from its static position, the buoy (3) will be displaced in the same direction, thus moving deeper whereby the buoyancy forces will increase, and so will participate to restoring forces on the floating unit (5),

- wherein the restoring force associated with the flexibility of the system is mainly horizontal such that the horizontal component of the restoring force represents at least 50% of the total restoring force,

- whereby the restoring forces on the floating units (5) result mostly from buoyancy forces at the buoys (3) such that the buoys (3) account for the major part of the mooring system flexibility,

- wherein the plurality of floating units (5) and the plurality of buoys (3) are, except at the border of the array, stationed in a configuration based on the repetition of a pattern, and

- wherein the repeated pattern is hexagonal and the six angles of the hexagons are filled alternatively with a floating unit (5) and a buoy (3).
Claim 2

The mooring system (10) of Claim 1 wherein the shared buoys (3) account for more than 50%, 75% or 85%, or even more than 90% or 95% of the incremental mooring system flexibility for any floating unit (5) of the system.
Claim 3

The mooring system (10) of Claim 2 wherein the buoys (3) to which each floating unit (5) is directly connected account for more than 50%, 75% or 85%, or even more than 90% or 95% of the incremental mooring flexibility for the floating unit (5).
Claim 4

The mooring system (10) of any of the preceding claims wherein the buoy (3) is at the surface or slightly below, positioned between a position piercing the water surface and a position deep enough to provide safe vessel sailing above the buoy and the slack lines.
Claim 5

The mooring system (10) of any of the preceding claims wherein the horizontal component of the restoring force represents at least 75 or 85%, or even at least 90 % or 95 % of the total restoring force.
Claim 6

The system (10) of any of the preceding claims wherein the buoy (3) is a spar buoy (3).
Claim 7

A mooring system (10) of any of the preceding claims, wherein the repeated pattern is such that a buoy (3) is connected to 2 to 6, preferably 3 to 6, even more preferably 3 floating units (5).
Claim 8

A mooring system (10) of any of the preceding claims, wherein the repeated pattern is such that each floating unit (5) is connected to 3 to 6 buoys, preferably 3 buoys (3).
Claim 9

The mooring system (10) of any preceding claim wherein the hexagons are equiangular.
Claim 10

The mooring system (10) of any preceding claim, wherein the floating units (5) are floating wind turbines and the array is a wind farm.