Support structures for offshore wind turbines

Abstract

The bottom-fixed offshore wind turbine installation includes a support column 10 landed on the seabed 40 in an upright orientation, resting on or slightly self-embedded into the seabed (e.g. a gravity

Description

This invention relates to foundation or support structures for offshore wind turbines. With the introduction of increasingly large offshore wind turbines that require correspondingly large subsea support structures, the invention aims to simplify, industrialise and de-risk such structures and the processes required to manufacture and install them. Offshore wind turbines can be fixed to the seabed but only in relatively shallow water of up to about 60m to 80m in depth. In deeper waters, offshore wind turbines typically float at the surface instead. As an example of the latter, FR 3079204 discloses a moored semisubmersible base or floater for a floating wind turbine. The semi-submersible floater design exemplified in FR 3079204, comprising a free-surface stabilised structure with a small draft, is currently dominant in the industry but is complex to fabricate. Tension leg platforms are also known to support floating wind turbines, where the floater is a buoyant hull that is anchored to the seabed by vertical tendons acting in tension. However, tension leg platforms are sensitive to anchoring systems, experience high loads and are unstable during installation. Another known floating wind turbine support comprises a floater in the form of a spar buoy, characterised by a slender buoyant body that is elongate along a vertical axis and so has a large draft. In this respect, US 9238896 and EP 1891328 disclose typical spar-supported floating wind turbines, moored in each case by taut lines or tethers. EP 2143629 shows that ensuring the stability of a spar-supported floating wind turbine may require additional taut mooring lines or tethers. Conversely, in EP 2410176, a spar buoy supporting a floating wind turbine comprises a bottom ballast tank for additional stability. JP 5738643 and JP 5727732 describe a hybrid concept in which a spar buoy is ballasted into an upright orientation at an assembly site and then lowered there in shallow water until a lower end of the buoy is landed temporarily onto the seabed under negative buoyancy. Conveniently, a wind turbine can then be installed onto an upper end of the landed buoy left protruding above the surface. The assembly of the buoy and the wind turbine is then refloated to lift the lower end of the buoy away from the seabed before the assembly is towed to an installation site in deeper water. There, the assembly is anchored via slack catenary mooring lines, with the buoy floating clear of the seabed and therefore serving as a conventional spar buoy throughout the operational life of the wind turbine. Thus, the buoy does not address operational challenges arising after the wind turbine has been united with the buoy and towed away from the assembly site toward the installation site. In this respect, floating wind turbines require large and costly floaters and, similarly, costly mooring or tethering systems. Their complexity and cost is difficult to justify in water that is shallower than about 120m. Indeed, shallower water may make it impractical to adopt a floater with a large draft, such as a spar buoy. Also, due to their complex motions in shallower water, floating wind turbines present challenges in relation to configuration of dynamic inter-array cables and calculation of design life. Yet, there is widespread demand for installing offshore wind turbines in intermediate water depths ranging between say 60m and 140m. The problem is that much of that depth range is too deep, technically, to adopt bottom-fixed structures and yet too shallow to employ conventional floaters economically. To address this need, the invention is concerned with wind turbine support structures that are fixed to the seabed, or bottom-fixed, but yet are capable of being deployed, economically, in intermediate water depths where conventional bottom-fixed and floating structures are impractical for different reasons. The optimal design of bottom-fixed support structures depends upon the size of the wind turbine, the characteristics of the seabed and the loads expected in service, especially bending or tilting due to horizontal forces and moments transmitted from the wind turbine due to water motion and wind gusts. Some bottom-fixed structures for supporting a wind turbine comprise multiple legs, for example jackets or tripods, where each leg is atop its own foundation. Typically, multi-leg structures are adopted to resist the high toads imparted by the larger offshore wind turbines that are now coming into service, and are suitable for waters of up to about 60m to 80m in depth. Other bottom-fixed structures are of a single-leg design such as a monopile, which are usually adopted in shallower waters of up to about 30m to 40m in depth. Offshore wind turbine support structures may be embedded into the seabed soil or into underlying rock. For example, a lower end of a monopile is typically driven into the seabed, which requires the wall of the monopile to be strong enough to penetrate the seabed and to withstand multiple driving impacts during installation. Of course, the composition of the seabed itself must also be amenable to penetration. In principle, a monopiie foundation can be anchored, supplementary, by taut mooring lines as proposed in www.entrionwind.com / technology and disclosed in WO 2023 / 141257 and WO 2022 / 086665. By combining fixed and floating platform technologies and restraining motion in all degrees of freedom, this solution is claimed to extend the capabilities of traditional monopile technology to water depths up to of 100m. However, there is still a need to drive the monopile into the seabed, which is all the more challenging in such deep water. An offshore wind turbine support structure may instead comprise one or more suction piles, also known as suction buckets or suction caissons. Gravity foundations are also known, in which the weight of the support structure, optionally supplemented by additional ballast material, is sufficient to fix the structure to the seabed. In WO 2021 / 070786, a bottom-fixed wind turbine is supported by an upright buoyantly-supported rigid column that is landed onto a pre-installed foundation on the seabed. The column can swivel about the foundation to pivot away from the vertical within a range of tilting movement permitted by slack catenary mooring lines. Conceptually, this is similar to US 6027286 in which a spar buoy of a hydrocarbon production system is attached to a seabed foundation, or foot, via riser pipes about which the buoy can pivot away from the vertical within limits also determined by catenary mooring lines. Another elongate columnar support for offshore wind turbines is proposed by AWC Technology (awctechnology.com). Its solution is described as an articulated column that defines a single-anchor buoyant foundation said to be suitable for very large turbines in water depths of 70m to 250m. An articulated joint at the base of the column provides compliancy with reduced forces, whereas buoyancy and hydrodynamic inertia provide stability. However, moorings are, explicitly, omitted to the benefit of through-traffic and fisheries. Against this background, the invention resides in a method of supporting a bottom-fixed offshore wind turbine, the method comprising: buoyantly supporting an elongate support column above a seabed; landing the support column on the seabed in an upright orientation; ballasting the upright support column before and / or after landing it; tethering the upright support column with inclined tensioned tethers that extend upwardly from the seabed to the support column; supporting the wind turbine atop the tethered upright support column; and generating electricity by operating the wind turbine atop the tethered upright support column. Ballasting the support column suitably comprises adding ballast below a centre of buoyancy of the support column and may comprise adding ballast when the support column is above the seabed and / or when the support column has been landed on the seabed. Conversely, the support column could be deballasted or subjected to increased buoyancy after being tethered. Positive buoyancy can be maintained above a centre of gravity of the support column. A selfuprighting moment can thereby be applied to the support column. Moreover, tilting of the support column can be restrained by virtue of horizontal force components applied to the support column by the tensioned tethers. A bottom end of the landed support column can be rested directly or indirectly upon the seabed, for example upon a pre-installed foundation structure that is interposed between the landed support column and the seabed. In these cases, horizontal movement of the bottom end of the support column with respect to the seabed can be opposed by friction. It is also possible for a lower portion of the support column to self-penetrate into the seabed, for example by applying suction at a bottom end of the landed support column. In that case, the resulting mechanical engagement between the bottom end of the support column and the seabed can resist horizontal movement of the support column relative to the seabed. The support column can be upended before being supported buoyantly above the seabed in the upright orientation, for example after being transported across water in a substantially horizontal orientation. At least a mast portion of the wind turbine could be upended with the support column. More generally, the wind turbine can be assembled or erected upon an above-surface portion of the tethered upright support column. Ballasting or buoyancy of the support column can be adjusted at any stage, for example after assembling or erecting the wind turbine to compensate for its weight. In the event of a tethering failure, the wind turbine can be supported with its inclination limited by buoyancy in an upward portion of the support column and ballast in a downward portion of the support column. Correspondingly, the inventive concept extends to a bottom-fixed offshore wind turbine installation that comprises: an elongate support column landed on a seabed in an upright orientation, the support column having buoyancy above a centre of gravity and ballast beneath a centre of buoyancy; inclined tensioned tethers that extend upwardly from the seabed to the support column; and an operational wind turbine mounted atop the tethered support column. The tethers may be mounted to the support column at a level offset toward a top of the support column, and can be inclined at an angle of say 30° to 75°, for example 30° to 60° to the vertical. A frictional interface at the bottom end of the support column can fictionally oppose horizontal movement of the bottom end of the support column with respect to the seabed. The upright support column may be held in frictional and / or mechanical engagement with the seabed by downward force applied by the tensioned tethers. Buoyant upthrust of the support column can exceed the combined weight load of the wind turbine, the support column and the ballast of the support column. Conversely, the combined weight load of the wind turbine, the support column and the ballast of the support column could exceed the buoyant upthrust of the support column. In any case, preferably, the buoyancy and the ballast of the support column are together sufficient to support the wind turbine against capsizing if the tethers should ever fail. In the invention, unlike the aforementioned prior art, the ability to land a spar-like support column on the seabed is not used merely for manufacturing or assembly before installation but especially also during normal power-generating operation of the wind turbine, after installation. The interaction between the support column and the seabed ensures stability against horizontal forces and counteracts moments generated by wind acting on the wind turbine above the surface. Thus, the invention embodies a principle, namely to exploit the opportunity of landing and mooring a support column of a formerly floating spar-like wind turbine in shallow water and then operating the wind turbine in that landed and moored configuration. The wind turbine is operated with the benefit of the additional stabilising contact provided by an interface between the landed floater and the seabed. Dynamics effects and associated amplification loads are reduced because the structure is quasi-static, being maintained in a stiff, substantially rigid position when in an energy production mode. Motions of the system, including heave especially, are restrained by landing the floater to achieve a substantially static configuration in which the system remains throughout the operational life of the wind turbine. The interaction between the support column and the seabed to resist horizontal slippage of the base of the support column during operation can be primarily or exclusively frictional. Nevertheless, that interaction could be supplemented by a minor degree of embedment of the support column into the seabed under self-weight or suction, hence providing an additional degree of mechanical engagement against horizontai slippage. Unlike monopiles of the prior art, however, the support column is not embedded to an extent that requires it to be driven or hammered into the seabed. Nor is the support column embedded to an extent that it can remain upright and embedded throughout its operational life without the additional bracing of moorings such as tethers. The inventive concept therefore contemplates a spar-like floater that is landed on the seabed at an installation location and so may be described as a landed spar. Once landed to serve as a support column, the floater is moored with taut inclined mooring lines, tendons or tethers to then support an offshore wind turbine generator throughout its operational life at the installation location. The support column rests on the seabed under self-weight supplemented by tension applied to the tethers. The support column therefore functions like a gravity base, but one whose load on the seabed can be controlled by adjusting the degree of negative buoyancy in the former floater, now landed and serving as a support column for a wind turbine extending above. The load of the support column on the seabed can also be controlled by adjusting tension in the tethers. Functionally, therefore, the invention envisages a synergistic combination of certain characteristics or attributes of a spar floater, in which ballast contributes to vertical stability, with certain attributes of a tension-leg platform and a gravity base to stop or limit motions of the floater after the floater has been landed and moored. This allows the system to enjoy an improved frequency response when the wind turbine is in an energy production mode. To a minor extent, the invention also shares an attribute of a traditional monopile but only so far as the bottom of the landed floater will interact with the seabed soil to some degree. However, none of these attributes alone govern the behaviour of the floater: the invention is not a spar, a tension-leg platform, a gravity base or a monopile but instead takes some of the functions and response modes of all such structures and applies them to different design load cases. In this way, the invention forms a bridge between bottom-fixed concepts that are traditionally used in shallower water and free-floating solutions that are traditionally used in deeper water. More specifically, when comparing the invention with conventional supporting structures used for offshore wind turbines: verticality of a spar relies principally upon hydrodynamics and ballast at the bottom of a floater; verticality and horizontal positioning of a tension leg platform relies upon very high tension in upright tendons; verticality of a monopile depends upon a tubular column making a stiff connection with deeply-penetrated soil; and verticality of a gravity base structure depends upon the inertia of heavy weights embedded or otherwise installed at the bottom the structure. In the invention, various aspects of conventional supporting structures combine with benefit: initially, spar functioning as a ballasted floater, and then after the floater is landed, using taut mooring connections in combination with reaction through contact of the bottom of the floater with the seabed to provide a permanent support for energy production above the surface. Moreover, the spar characteristics of the landed floater provide a failsafe characteristic in the event of mooring failure. When compared with a conventional spar solution, for example, a floater of the invention can support a larger wind turbine because soil contact interaction in combination with taut moorings ensures the stability of the landed floater in an operational mode. In contrast, a spar buoy relies upon its displacement and hydrodynamics and so requires the buoy to be larger relative to the wind turbine that it supports, in this respect, the floater of the invention is not a spar buoy in the sense that the floater geometry and components need not be designed to satisfy all functions in ail design load conditions on their own. Landing and tethering allows a smaller floater to fulfil the same performance criteria as a larger conventional spar buoy. The invention also considers the contribution of tension loads in the tethers. Vertical components of those tension loads minimise the required mass of ballast applied to the floater, whereas horizontal components of those tension loads reduce tilting of the wind turbine in operation. Thus, landing and inclined taut mooring are highly complementary to the floater function, helping to handle vertical loads and to ensure primary stability during operation of the wind turbine. In general, therefore, the invention benefits from a combination of hydrostatic stiffness, tendon or tether stiffness and stiffness of the connection to the seabed. This allows an optimal solution in terms of mass, static stability and modal analysis, enabling a lighter structure to support a wind turbine in greater water depths than bottom-fixed solutions allow and to do so more economically than the alternative of purely floating solutions. Thus, the invention proposes an offshore structure for supporting a wind turbine in an offshore environment. The structure comprises an elongate floater that is grounded on the seabed in an upright orientation. The grounded floater is connected to mooring lines or tethers that are distributed around the floater and tensioned to resist wave, current and wind loadings. The tethers combine with interaction at the interface between the bottom of the landed floater and the seabed, hence ensuring station-keeping and stability of the landed floater and the wind turbine in the offshore environment. In examples to be described, the floater has a body of an elongate cylindrical or tubular shape, possibly with different diameters along its length. For example, the floater may comprise a monotube, like a conventional monopile, with a single thick tubular wall. The floater could instead comprise a bundle or group of tubes connected together in parallel or a polygonal-section tube with stiffeners to maintain its integrity. The floater can be made of steel or concrete tube or a mix of steel, concreted plastic or composite tubes, assembled in a bundle. The length of the floater is greater than the depth of the water in which it is deployed, for example from 70m up to 200m water depth. The bottom end of the floater thereby touches the seabed in a permanently-installed configuration and a top part of the landed floater emerges out of sea, being aligned to receive the vertical mast of a horizontal-axis wind turbine. Thus, the top part of the landed floater supports a connection with the wind turbine above sea level, such as a transition piece, and can also support a cantilevered work platform. To this extent, the landed floater interacts with above-surface structures in a manner similar to conventional monopiies or jackets used in bottom-fixed offshore wind turbines. Thus, conveniently, a conventional wind turbine can be mounted atop the landed floater. As the support column is defined by a floater as noted above, it is possible to control its buoyancy to reduce the apparent weight of the assembly of the wind turbine and the support column in comparison to a conventional bottom-fixed solution fora similar wind turbine. In other words, adjusting the buoyancy of the support column can control the weight load that is exerted by the foot of the support column on the seabed when in operational service. The weight load should be sufficient for the foot of the support column to engage the seabed against lateral slippage but not so great that the seabed cannot provide adequate support. In principle, therefore, the invention could be used where the seabed composition would be inadequate to support the weight of a conventional bottom-fixed solution. More generally, a majority of the weight of the system including the wind turbine can be taken by the residual buoyancy of the support column and a minority of the weight of the system can be taken by the reaction where there is contact between the support column and the seabed, hence reducing interaction with and penetration into the seabed soil. Interaction with the seabed soil can be addressed in various ways, depending on the local geology; the invention makes it possible to control the degree of interaction with soils, reducing design risk relating to the soil and hence to that interaction. The connection or contact between the landed floater and the seabed soil can be direct or via an intermediate structure, for example by landing the bottom of the floater on a template, by seating it on a mudmat or by connecting it to a stiff pile. An area of the seabed can be prepared before landing the floater on top of the prepared area, for example by compacting the soil or by dumping or embedding rocks. A degree of penetration of the landed floater into the seabed soil can be accepted or promoted, possibly including some suction-assisted penetration into the soil by virtue of a suction pile or suction caisson disposed at the bottom end of the floater. In normal operational conditions of the wind turbine, being generation of electricity up to its upper limit of function, the tethers mainly contribute an uprighting moment to fulfil the verticality requirements for the wind turbine. The reaction point where the support column touches the seabed also contributes to stability. In essence, the mast of the wind turbine must be maintained within than a few degrees of verticality to ensure that the blades of the wind turbine are presented efficiently to the incident wind. Thus, in addition to restraining vertical movement or heave of the structure, the tethers contribute to stabilising the structure in the horizontal plane, keeping station by counteracting horizontal loads in the marine environment caused by waves, currents and winds. The taut tethers will experience significant tension and may advantageously be pretensioned during installation. However, the tethers pull the landed floater toward the seabed with a significant inclination to the vertical to ensure horizontal station-keeping response in addition to vertical stability as per production criteria. Their inclination allows the tethers to counteract horizontal environmental loads more effectively than the substantially vertical tendons of a tension leg platform, thus reducing the tension that is required for inherent stability. The floater that serves as the support column is intended to be towed horizontally from its launching port or fabrication yard before then being upended and landed at the installation site in an appropriate water depth. Upending may be performed by ballasting the floater in an internal compartment or tank, by releasing some temporary buoyancy if the floater is already charged with ballast, or by adding counterweights to cause the floater to upend passively Cranes or winches may be involved to control upending but are not intended to be the primary driver of the upending process. In partially-ballasted conditions, when carrying some water or solid ballast near its bottom end to keep its centre of gravity, or centre of mass, below its centre of buoyancy, the upright floater can support itself buoyantly off the seabed before being landed. The floater can even be towed stably in an upright orientation, hence not necessarily requiring upending. Even before the mooring tethers are installed and tensioned, the landed upright floater remains stable on its own in minor environment conditions. Stability may be ensured by adding solid ballast or water ballast to the floater when in place at the installation site, before or after installing and tensioning the tethers. After the floater has been landed on the seabed and supported with tensioned mooring tethers, the wind turbine is then installed on top. Conveniently, this can be done in a manner similar to current bottom-fixed solutions, possibly using a jack-up vessel or a floating heavy lift vessel such as a large semi-submersible crane vessel. Electrical components that are not appropriate to be installed before upending can be integrated as part of the wind turbine, for example being placed with the bottom part of the wind turbine mast Alternatively, such components can be installed in a module, in a canister or as part of a transition piece, or can be retrofitted by lifting them afterwards. If the assembly of the wind turbine and the support column should ever break free from its moorings accidentally, for example due to failure or detachment of one or more of its tethers in a storm, the assembly can behave as a floating spar-type arrangement due to its inherently buoyant floating design including stabilising ballast located beneath the centre of buoyancy of the structure. This ensures its survival, meaning that the wind turbine remains stable with its mast maintained at a reasonable angle to the vertical, for example up to 20° from vertical, even in exceptional multiannual (fifty-year or hundred-year) storm conditions.. Thus, in the event of mooring failure, the system of the invention is designed not to capsize. Instead, the system reverts to a spar function with degraded criteria but remaining safe, preserving the integrity of the wind turbine in a parked or survival mode. In this respect, the support column is not a monopile whose verticality and uprighting moments are ensured solely by a stiff connection with embedding soil. Nor is the support column a taut or guyed tower whose survival depends upon being fixed to the seabed with the support of cables. Instead, verticality and uprighting moments are ensured primarily by the spar-like configuration of the floater, being ballasted at the bottom and with buoyant self-uprighting characteristics to the extent that the floater is vertically stable in passive and low environmental conditions. The floater is therefore capable of maintaining the integrity of the system in accidental conditions with broken moorings or tethers, in which the system will remain afloat or landed with an acceptable degree of tilt or heel. Embodiments of the invention implement a method for installing and exploiting a spar-type wind turbine, the method comprising: providing the spar-type wind turbine, comprising a floatable spar body that can serve as a support column, a foot at the bottom of the spar body and interfaces for mooring lines; defining three modes of exploitation, the modes being: floating mode, in which the spar body is floating or otherwise buoyantly supported and extends through the sea surface; fixed mode, in which the spar body is ballasted into negative buoyancy to lay its foot in contact with the seabed; and moored mode, in which the spar body is anchored to the seabed by tensioned mooring lines; and combining and changing the modes during the life of the wind turbine to ensure the stability of the wind turbine in different conditions. Advantageously, the spar-type wind turbine of the invention is floatable and re-floatable. The method of the invention is apt to be implemented in a water depth of 50m to 200m, for example. Usefully, this embraces the intermediate water depth range in which neither conventional fixed nor floating wind turbine solutions may be fully appropriate. The interface between the foot and the seabed may involve simple contact either directly with soil or rock of the seabed or ensured by an intermediate foundation that is pre-installed at the seabed. The foot can have a simple flat surface at the interface, without holes or recesses. Normal operation of the wind turbine may involve a combination of the fixed and moored modes, whereas degraded operation may involve the floating mode or the moored mode. The stability of the wind turbine in normal operation can be ensured by the slightly negative buoyancy of the spar body, or of the assembly of the spar body and the wind turbine, combined with contact friction between the foot and the seabed or a seabed foundation, and tensioning of the mooring lines. In summary, a bottom-fixed offshore wind turbine installation of the invention comprises a support column that is landed on the seabed in an upright orientation, resting directly or indirectly on the seabed or slightly self-embedded into the seabed. The support column has buoyancy above its centre of gravity and ballast below its centre of buoyancy. Inclined tensioned tethers extend upwardly from the seabed to the support column. An operational wind turbine is supported atop the tethered support column, ready to generate electricity for export via connected cabling. The support column can be towed horizontally, uprighted and held buoyantly above the seabed before being ballasted, landed, optionally further ballasted and then tethered before assembling or erecting the wind turbine on top. Buoyancy and / or ballasting of the tethered support column and / or tensioning of the tethers can be adjusted to control interaction with the seabed soil. Buoyancy and ballasting of the support column can also cooperate to prevent the installation capsizing in the event of mooring failure. In order that the invention can be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic side view of a support column of the invention for supporting an offshore wind turbine; Figure 2 is a schematic side view of the support column of Figure 1 being surface-towed as a floater toward an offshore installation site, in a horizontal orientation; Figures 3a to 3d are a sequence of schematic side views that show the support column being upended, landed on the seabed in an upright orientation and moored to the seabed at the installation site; Figure 4 is a schematic side view that shows the option of landing the support column on a foundation pre-installed on the seabed at the installation site, before the support column is moored; Figures 5a and 5b are a sequence of schematic side views that show another option of embedding the support column into the seabed at the installation site with the assistance of suction, before the support column is moored; Figure 6 is a schematic side view that shows an offshore wind turbine being assembled atop the moored support column at the installation site; Figure 7 is a schematic side view that shows the support column being upended with a tower or mast of the wind turbine already attached; Figure 8 is a schematic side view that shows the offshore wind turbine of Figure 6 now completed and in operation when generating electricity at the installation site; and Figure 9 is a schematic side view that shows the assembly of the support column and the offshore wind turbine supported buoyantly with controlled inclination in an emergency situation foilowing failure of the moorings. Referring firstly to Figure 1 of the drawings, a subsea support column 10 of the invention is exemplified here by an elongate monotube of circular cross-section, whose side wall is rotationally symmetrical around a central longitudinal axis 12. The support column 10 could have substantially constant diameter along its length but in this example tapers upwardly by virtue of a narrower top portion 14 that is joined to a wider bottom portion 16 by a frusto-conical intermediate portion 18. By way of example, depending upon the targeted water depth, the support column 10 may be 100m to 150m long and 10m to 15m wide at its base. The bottom end of the support column 10 is closed by a flat end wall 20 in this example, lying in a plane that is orthogonal to the central longitudinal axis 12. The hollow top portion 14 of the support column 10 can be open or may be closed by a bulkhead or a removable plug, at least temporarily for transport purposes. An array of mooring mounts 22 are spaced angularly around the side wall of the support column 10 at an upwardly-offset position located toward its upper end. In this example, four equi-spaced mooring mounts 22 are provided but more or fewer mooring mounts 22 could be present in other examples. The support column 10 contains one or more ballasting tanks. A single ballasting tank 24 is represented here schematically in dashed lines but that tank 24 could be partitioned into separate chambers. Alternatively, there could instead be multiple ballasting tanks, for example disposed in longitudinal series along the support column 10. Other than its ballasting provisions, the support column 10 is of similar construction to a monopile and so can be fabricated at a shipyard using techniques that are familiar in the art. Conveniently, the support column 10 can be fabricated in a horizontal orientation: moreover, to minimise its draft to suit shallow inshore water, the support column 10 can also be launched into and transported through water in that orientation. Thus, there is no need for deep-water port or fjord to construct and transport the support column 10. In this respect, Figure 2 shows a tug 26 towing the support column 10 toward an installation site. In this example, the support column 10 is towed across the surface 28 of the sea in a horizontal orientation. Ballast 30, conveniently water, is shown in the ballasting tank 24 to adjust the draft of the support column 10 for stability, while maintaining positive buoyancy overall to keep the support column 10 at the surface 28. The ballast 30 could be shifted longitudinally to adjust the trim of the support column 10 if needs be. Conveniently, as shown, the mooring mounts 22 can be used to attach towing lines 32 to the support column 10. On arrival at the installation site, the support column 10 is upended as shown in Figures 3a and 3b. Although not shown here, the tug 26 of Figure 2 can remain connected to the support column 10 to control upending. Conveniently, upending can be initiated by adding or transferring ballast 30 toward the lower end of the support column 10 as shown. The support column 10 therefore contains a positively-buoyant section 34, which may contain air, atop a negatively buoyant section containing ballast 30. The result of this downwardly-offset ballasting, as shown in Figure 3b, is that the support column 10 eventually lies stably in an upright orientation assured by its centre of gravity 36 being held beneath its centre of buoyancy 38. The length of the support column 10 exceeds the water depth at the installation site. Initially, however, enough buoyancy is maintained in the upright support column 10 for the bottom of the support column 10 to be held suspended clear of the seabed 40 like an unmoored spar buoy as shown in Figure 3b. This allows fine control of the position of the support column 10 in a horizontal plane to ensure that the support column 10 will be installed accurately. Once correctly positioned, the support column 10 is ballasted further, for example by admitting more water ballast 30 into the ballasting tank 24 as shown in Figure 3c. This causes the support column 10 to sink until its bottom end wall 20 encounters and comes to rest on the seabed 40, leaving only part of the top portion 14 of the support column 10 still breaking the surface 28. On landing the support column 10 in this way, frictional and mechanical engagement of its bottom end wall 20 with the seabed 40 then resists lateral movement of the support column 10 relative to the seabed 40. Additional ballast 30, such as solid ballast 30A that is denser than water, can be added beneath the centre of buoyancy 38 to increase stability as shown in Figure 3d. Moreover, residual positive buoyancy in the upper section 34 of the support column 10 provides uprighting support. This keeps the support column 10 self-stable in a substantially vertical orientation despite horizontal forces of currents, waves and winds acting on the support column 10 at levels beiow, at and above the surface 28. The inherent stability of the ballasted support column 10 facilitates subsequent installation operations. The residual buoyancy of the support column 10 also supports much of its weight, such that the apparent weight load exerted on the seabed 40 is substantially less than the actual weight of the support column 10 including its ballast. Figure 3d shows the support column 10 now moored by attaching tethers 42 to respective ones of the mooring mounts 22. Each tether 42 extends radially from the support column 10 in plan view. The tethers 42 splay downwardly and outwardly from the mooring mounts 22 with mutually-opposed inclination in a frusto-conical arrangement at an angle of, for example, between 30° and 75° to the vertical. Each tether 42 terminates in a respective anchoring foundation 44, such as a suction pile, a pin pile, a clump weight or a drag anchor. The anchoring formations 44 are embedded in or lie on the seabed 40 at locations spaced horizontally from the support column 10 and from each other. The inclined tethers 42 are then post-tensioned as shown in Figure 3d. The applied tension substantially exceeds that arising from self-weight of each tether 42. The tension in the tethers 42 applies horizontal and vertical force components to the support column 10. The horizontal force component resists or compensates for tilting of the support column 10 away from the vertical: conversely, the vertical force component pulls the support column 10 down against the seabed 40. The downward force exerted by the support column 10 on the seabed 40 can be adjusted by adjusting the ballasting or buoyancy of the support column 10. As shown In Figure 3d, this could include reducing ballast 30 in the lower section and / or increasing buoyancy in the upper section 34 once the tethers 42 have been installed. In principle, adjusting the tension in the tethers 42 can have a similar, if lesser, effect. Also, reducing ballast 30 and / or increasing buoyancy in the upper section 34 of the support column could have the effect of applying tension to the tethers 42. Further adjustments of ballasting or buoyancy can be made when a wind turbine is erected atop the support column 10, as will be described below. Figure 4 shows the possibility of landing the support column 10 on a previously prepared area of the seabed 40. The seabed 40 itself can be prepared to bear the downward load of the support column 10, for example by rock dumping. However, in this example, the seabed 40 is prepared by pre-installing a subsea foundation 46 on which the support column 10 will rest, with the foundation 46 then being interposed between the support column 10 and the seabed 40. By way of example, the foundation 46 could be a rigid raft or template structure lying on or embedded into the seabed 40, or a mudmat Tethers 42 are then installed as shown in Figure 3d. Figures 5a and 5b show another variant, in this case an adaptation of the bottom of the support column 10 to assist self-penetration promoting mechanical engagement with the seabed 40. In this example, that adaptation is a downwardly-opening suction caisson formation 48. As is conventional, the surrounding skirt of such a formation 48 initially penetrates the soil of the seabed 40 under self-weight to form a suction chamber as shown in Figure 5a from which water is then pumped out to draw the support column 10 deeper into the seabed 40 as shown in Figure 5b. This is to be distinguished from a monopile that is hammered into the seabed 40 in a pile-driving operation. Again, tethers 42 are installed subsequently as shown in Figure 3d. In all of the preceding embodiments, the landed and tethered support column 10 is then ready for a wind turbine 50 to be assembled atop the upper portion of the support column 10 that projects above the surface 28. In this respect, Figure 6 shows a floating crane 52 assembling a mast 54 of the wind turbine 50 from successive mast sections 56 fixed together end-to-end. The mast 54 is aligned with the central longitudinal axis 12 of the support column 10, and may be received in the open upper portion 14 of the support column 10 serving as a socket. An intermediate structure such as a transition piece could be installed between the support column 10 and the mast 54. Figure 7 shows the alternative possibility of integrating at least part of the wind turbine 50, for example at least part of the mast 54, with the support column 10 and upending that entire assembly after transporting the assembly to an installation site, in this case, it may be advantageous to adjust the trim of the support column 10 to incline the central longitudinal axis 12 to the horizontal, thus holding at least some components of the wind turbine 50 clear of the surface 28 during transportation. Figure 8 shows the wind turbine 50 completed with a nacelle 58 and rotor 60 atop the completed mast 54 and in power-generating operation as the rotor 60 turns in the wind. Notably, the support column 10 remains in its landed and tethered configuration throughout the operational life of the wind turbine 50. The residual self-righting buoyancy 60 and ballasting 62 of the support column 10, the tension 64 in the tethers 42 and the frictional and / or mechanical engagement 66 between the bottom of the support column 10 and the seabed 40 ensure that the mast 54 of the wind turbine remains within an acceptable angle to the vertical under horizontal loadings of waves, currents and winds. Finally, Figure 9 shows a further advantage of the residual self-righting buoyancy 62 and downwardly-offset ballasting 64 of the support column 10, namely that if the moorings provided by the tethers 42 should ever fail in a storm, the assembly of the support column 10 and the wind turbine 50 will not capsize. Instead, the restoring moment created by the couple forces of buoyancy 62 and ballasting 64 ensure that the mast 54 of the wind turbine 50 will remain within a survivable and recoverable inclination of up to, say 20° to the vertical as shown. Figure 9 shows the support column 10 floating clear of the seabed 40 but the bottom of the support column 10 could instead rest on the seabed 40, depending upon the balance between the overall buoyancy and weight of the system. Many other variations are possible within the inventive concept. For example, the support column, with or without at least part of the above-surface wind turbine structure, could be transported to an installation site aboard a barge or a semi-submersible vessel. Also, control of the orientation and depth of the support column could be effected by adding or removing external ballasting or buoyancy. Optionally, a crane or winch can act on the top portion of the support column to assist or to control the upending process. However, it is preferred that ballasting is the sole or primary driver of upending. 5 The bottom of the support column could have various formations to promote self-penetration and mechanical engagement with the seabed, such as one or more downwardly-tapering blade or spear formations.

Claims

1. A method of supporting a bottom-fixed offshore wind turbine, the method comprising:buoyantly supporting an eiongate support column above a seabed;ballasting the upright support column;landing the support column on the seabed in an upright orientation;tethering the upright support column with inclined tensioned tethers that extend upwardly from the seabed to the support column;supporting the wind turbine atop the tethered upright support column; andgenerating electricity by operating the wind turbine atop the tethered upright support column.

2. The method of Claim 1, wherein ballasting the support column comprises adding ballast below a centre of buoyancy of the support column.

3. The method of Claim 1 or Claim 2, wherein ballasting the support column comprises adding ballast when the support column is above the seabed.

4. The method of any preceding claim, wherein ballasting the support column comprises adding ballast when the support column has been landed on the seabed.

5. The method of any preceding claim, comprising deballasting or increasing buoyancy of the support column after tethering the support column.

6. The method of any preceding claim, comprising maintaining positive buoyancy in the support column above a centre of gravity of the support column.

7. The method of Claim 6, comprising applying a self-uprighting moment to the support column by virtue of said positive buoyancy.

8. The method of any preceding claim, comprising restraining tilting of the support column by virtue of horizontal force components applied to the support column by the tensioned tethers.

9. The method of any preceding claim, comprising resting a bottom end of the landed support column directly or indirectly upon the seabed.

10. The method of Claim 9, comprising resting the bottom end of the support column upon a pre-installed foundation structure that is interposed between the landed support column and the seabed.

11. The method of Claim 9 or Claim 10, comprising frictionally opposing horizontal movement of the bottom end of the support column with respect to the seabed.

12. The method of any of Claims 1 to 8, comprising self-penetrating a lower portion of the support column into the seabed.

13. The method of Claim 12, comprising applying suction at a bottom end of the landed support column.

14. The method of Claim 12 or Claim 13, comprising mechanically engaging the bottom end of the support column against horizontal movement relative to the seabed.

15. The method of any preceding claim, comprising upending the support column before buoyantly supporting the support column above the seabed in the upright orientation.

16. The method of Claim 15, preceded by transporting the support column across water in a substantially horizontal orientation.

17. The method of Claim 15 or Claim 16, comprising upending at least a mast portion of the wind turbine with the support column.

18. The method of any preceding claim, comprising assembling or erecting the wind turbine on an above-surface portion of the tethered upright support column.

19. The method of Claim 18, comprising adjusting ballasting or buoyancy of the support column after assembling or erecting the wind turbine.

20. The method of any preceding claim, further comprising, foilowing a tethering failure, supporting the wind turbine with its inclination limited by buoyancy in an upward portion of the support column and ballast in a downward portion of the support column.

21. A bottom-fixed offshore wind turbine installation, comprising:an elongate support column landed on a seabed in an upright orientation, the support column having buoyancy above a centre of gravity and ballast beneath a centre of buoyancy;inclined tensioned tethers that extend upwardly from the seabed to the support column; andan operational wind turbine supported atop the tethered support column.

22. The installation of Claim 21, wherein the tethers are mounted to the support column at a level offset toward a top of the support column.

23. The installation of Claim 21 or Claim 22, wherein the tethers are inclined at an angle of 30° to 75° to the vertical.

24. The installation of any of Claims 21 to 24, wherein a bottom end of the support column lies directly or indirectly upon the seabed.

25. The installation of Claim 24, wherein the bottom end of the support column rests upon a pre-installed foundation structure that is interposed between the landed support column and the seabed.

26. The installation of any of Claims 21 to 25, comprising a frictional interface at the bottom end of the support column, frictionally opposing horizontal movement of the bottom end of the support column with respect to the seabed.

27. The installation of any of Claims 21 to 24, wherein a lower portion of the support column is self-penetrated into the seabed.

28. The installation of Claim 27, comprising a suction formation at a bottom end of the support column.

29. The installation of any of Claims 21 to 28, wherein the upright support column is held in frictional or mechanical engagement with the seabed by downward force applied to the support column by the tensioned tethers.

30. The installation of any of Claims 21 to 29, wherein buoyant upthrust of the support column exceeds a combined weight load of the wind turbine, the support column and the ballast of the support column.

31. The installation of any of Claims 21 to 29, wherein a combined weight load of the wind turbine, the support column and the ballast of the support column exceeds buoyant upthrust of the support column.

32. The installation of any of Claims 21 to 31, wherein the buoyancy and the ballast of the support column are together sufficient to support the wind turbine against capsizing if the tethers fail.Application No: GB2319861.7Claims searched: 1-2023Examiner:Mr Tom SimmondsDate of search: 6 June 2024Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1-11, 18-20 US 2010 / 0150665 Al (KARAL) See Figures noting column floated to site, ballasted by filling internal cavity to rotate to upright condition, and tethered by inclined tether before turbine is fixed in place. X 1-9,11, 18-20 US 2012 / 0045345 Al (HORTON et al.) See Figures 11-14 and para. 43 in particular, noting support column 160 having fillable partitioned tank 116 for ballasting and titling the column on installation, for gravity fixing, later secured with tethers. X 1-11, 15-20 US 11739732 B2 (MAHER et al.) See whole doc. noting column with 'air-over-water' fillable ballast tank for upending column, wherein the buoyant towers may sit on bases 12a-c with tensioned lines 12. X 1-11, 18- 20 US 2012 / 0093589 Al (BROUGHTON et al.) See Figures noting adjustable ballast support column, which can be floated off vessel 78 and ballasted to upend and fix to seabed, with tethers 92 added, the support having a restoring force due to relative centre of mass and buoyancy (para. 63). X 1-14, 18-20 WO 2023 / 141257 Al (ENTRION WIND INC) See whole doc noting column driven into seabed (landed) with tensioned tethers.Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if combined with one or more other documents of same category. P Document published on or after the declared priority date but before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:Search of GB, EP. WO &US patent documents classified in the following areas of the UKCX :Worldwide search of patent documents classified in the following areas of the IPC____________E02B; E02D; F03D______________________________________________The following online and other databases have been used in the preparation of this search reportSEARCH - PATENTInternational Classification:Subclass Subgroup Valid From E02B 0017 / 02 01 / 01 / 2006 E02D 0023 / 02 01 / 01 / 2006 E02D 0027 / 52 01 / 01 / 2006

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